Siegel grinned with satisfaction.

His message to Wall Street was crisp and clear: People were paying way too much for Internet stocks. Sell them. "The buck must stop somewhere," he had written. "Eventually the big Internet companies must convert all this Monopoly money into hard earnings, or their prices will collapse."

And collapse - at least for the moment - they did.

That day, Internet stocks opened sharply down. By noon, disaster was looming for the cream of the Internet companies, which were to suffer their worst single day of trading ever.

Amazon.com, the online retailer, fell $31.06 to $158.94 a share. Ameritrade, an online trading service, plunged $38.06 to $87.88. Cnet Inc., an online news service, dropped $34 to $86. Even America Online, one Internet company actually earning profits, felt the sting of Siegel's poisonous pen. It lost $23.88 to close at $158.88.

That evening Siegel, 53, was on CNN, CNBC and NPR, as interviewers laid the Internet stock rout at his feet. In Chicago the next day, Siegel joked: "I'm glad I'm not traveling to Silicon Valley. I would have been stoned in the street."

But a more sober view came from Robert Shiller, a Yale economist and longtime friend of Siegel's: "It's kind of frightening the power he has obtained. He made billions of dollars disappear with his words."

In this era of the financial analyst as celebrity, Siegel - a bookish-looking Wharton professor with thick glasses and a nasal Chicago accent - has star power. He's got a book that's selling well, he makes speeches for as much as $15,000 a pop, and he's on the Rolodex of financial reporters from New York to London to Tokyo.

Competitive and relishing the spotlight - he keeps a scrapbook of his articles - he has fine-tuned his game of cat-and-mouse with the financial media. One day in June, when an MSNBC news director called him for an interview about interest rates, Siegel politely declined. Later in the day, he explained, "They're not big enough." His wife, Ellen Schwartz, teases that one of his favorite words now is extensively, as in, "I was quoted extensively in the New York Times today." He replies, a little sheepishly, "Extensively is at least twice."

And Siegel's influence is not limited to journalists. Last summer, Federal Reserve Chairman Alan Greenspan asked Siegel and several other financial experts to Washington to discuss the valuation of the stock market.

Five years ago, Siegel wrote the book that put him on the map - Stocks for the Long Run: The Definitive Guide to Market Returns and Long-Term Investment Strategies. He had a compelling, though controversial, theme: Buy stocks. Buy lots of stocks. Don't put your money in government bonds. Don't put your money in gold. Don't even put your money in the bank, if you're investing long-term.

In one scenario, using an investor who planned to keep his money in stocks for 30 years, Siegel calculated that it was safer to borrow money on margin and put it into the stock market than to buy government bonds. This was a very risky proposition to conventional ears, and one that in a crash has sunk many investors.

Stocks for the Long Run was projected by its publisher to sell at most 6,000 copies. After all, Siegel was untested and unknown as an author. The book made no wild projections and offered no get-rich-quick strategy.

Siegel's narrative focused on comparing stock returns to government bonds and gold dating to 1802, "further back than anyone has gone before," he says. It was a neat trick. Even today, he makes it sound as if he visited Delphi and spoke with the Investment Gods themselves.

In reality, Siegel had stumbled across the stock-return data in an academic journal in the early 1990s. He saw a way to package the information for a wide audience, though the writing wasn't easy. The first page took 80 attempts. "It was horrendous to watch," Schwartz says. "We were at the Shore at the house, and he couldn't think about relaxing. He never really left the house for two weeks. He was feeling under a lot of pressure."

The first printing sold out; thousands more were ordered. So far, 135,000 copies of Stocks for the Long Run have been sold.

Its greatest marketing tool has been the stock market itself, which has more than tripled in value since the book was published. Siegel has become something of a cult hero with investors and financial planners, although some academics believe he has overstated the case for stocks.

"The notion that you can always guarantee that stocks can outperform bonds, I don't believe that," says G. William Schwert, an economics professor at the University of Rochester who initially published the historic stock market returns Siegel used for his book. "You might think of the U.S. as the best possible draw. I mean, there have been a lot of wars this century, but none of them have ever been fought here. Things are rosier than they should be. . . . But you can't get carried away."

So far, share prices have held up and Siegel looks brilliant. So he's in demand. But optimism has reached precarious heights. After five years of unceasing advances, even Siegel is talking about caution.

It's a Wednesday morning on the Wharton campus, and Siegel, a little disheveled, is dreamily looking out on Locust Walk from his new second-floor office in Steinberg Hall-Dietrich Hall. It's an indicator of his prominence - and his seniority - that he has 30 percent more room than before and four windows instead of one.

"I think I pulled a muscle in my back," he says. Maintenance workers didn't have the time to move him, so he recruited his wife - "she's much better at organizing things than me; I'd just spread the stuff out."

For all of his celebrity - and the extra cash that comes along - Siegel has changed very little over the years, friends and family say. He still drives the family's only car, a six-year-old green Buick LeSabre, with 42,000 miles on the odometer. Many days, he takes the subway to University City.

Siegel meets a group of friends once a month at the Marathon Grill. "It's a lot if the bill comes to $5 apiece," says one of them. Siegel, his wife and two sons have vacationed at the same rental for more than a decade.

Schwartz says she and her husband do what they've always done. Dine out, go to the ballet and theater, collect art glass. He travels a lot now, though, and they argue about that.

"What I think is most remarkable about him is his stamina," she says. "I guess you expect me to think it would be his brilliant ideas. But it's the stamina that he has. To get out there to go on tour, travel as much as he does, to take all those calls from New York. He's always coming up with new ideas. "

When he travels on business, Siegel takes along a laptop computer and almost no luggage. He shows up at the airport just as the jet is about to pull out of the gate. On trips to the West Coast, the only suit he takes is the business suit he's wearing.

"We sometimes wonder what will happen to his one suit," says John Waterman, a managing director for Rittenhouse Financial Services Inc. in Radnor, which pays Siegel a speaking retainer. "It's a little bit stressful to us, but that's the way he is." Waterman says the company puts up with his eccentricities because Siegel still has drawing power.

Allen Domb, the city's leading condominium salesman, was selling door locks and hoping to break into real estate when he met Siegel, a recently tenured professor, 20 years ago.

"He can do mortgage calculations in his head," Domb says. "It is pretty amazing to see him give you a rate of return on a property. He'll say, 'Well, what is it selling for and what will the rent be?' And within about five seconds he'll say, 'Well, that's a return of -.' And I haven't even had a chance to put the information in my calculator. "

Another friend, Henry Donner, a Center City lawyer, cracks, "He never leaves enough for the tip, so he can't be that good with numbers. "

Even when he was a boy, numbers and graphs fascinated Siegel. He tells of a morning glory vine outside the family's ranch house in the Chicago suburb of Highland Park. When he was about 7, he would count the new blooms each morning, charting them on graph paper.

In third grade, Siegel found the largest number he'd ever heard of in the dictionary: vigintillion. It's a 1 followed by 63 zeros. Intrigued, Siegel decided to see how long it would take to reach. He doubled 2 to get 4, 4 to get 8, 8 to get 16, writing the numbers on strands of adding machine paper, until he reached a vigintillion. Then Siegel showed his handiwork to his teacher. She didn't know what to say.

"Some people are good with crossword puzzles," Siegel says. "Some people are good with words, you know. Well, I was very good with numbers. "

In Columbia University, he studied, not surprisingly, mathematics. But he yearned to apply math to a broader topic and eventually enrolled in economics classes. When not in class, he wandered the city looking at the skyscrapers. Tall buildings always excited him.

He also found himself drawn to Wall Street. "I would take the subway down to the New York Stock Exchange and watch the trading," Siegel says. "I used to do that every week. I was fascinated. I was there at the close when they rang the bell, you know. It was the fascination with the market. Not so much individual stocks, but the market. "

In letters home, he'd tell his mother about what he was doing and typically would include "his take" on the stock market.

It's a Friday in mid-July, and Siegel is making another lecture to financial planners, this time at the Marriott in Center City. Outside the weather is blazing hot. He notices some people sneaking out of his talk. Have they heard this all before? Are the ideas too stale? He is a man who pays attention to the crowd. A few people stick around to press him on the buzz: Now that the stock market has hit 11,000, does he believe in the wildest of projections - 35,000 points within a few years? "I'm not in that camp," Siegel says pointedly.

Siegel is more and more uncomfortable with the lofty stock prices. Years ago, telling people stocks would rise was a "no-brainer," he says. Now he believes stock gains will slow to their historic levels of about 7 percent a year, maybe slightly higher if the world economy expands and technology companies grow the way they have been.

He's painfully aware that history is loaded with Wall Street forecasters who made themselves look foolish with their predictions. Yet peddling caution won't sell books or insight.

"In my talks now, I make a point of saying that I am not a person who thinks the stock market is always a buy," Siegel says. "It can be overpriced. It can get to the point where you should not invest. I do not believe we are at that point yet. But can we get there? Is it possible? Most certainly."