The timing reflects the height of subprime lending in the summers of 2005 and 2006, when many borrowers secured loans scheduled to adjust in two or three years. For many, that adjustment means their interest rate will rise two to three percentage points.

"The next six months, the industry, all of the folks that are out there trying to solve this problem, they are going to be very busy," said Mark Fleming, chief economist for First American CoreLogic Inc., a California research firm. "There are a lot of people facing their resets right now. A good share of them don't have the refinance option."

Nationally, the number of subprime adjustable-rate loans resetting peaked last month at 7.61 percent of the loans outstanding, according to data from CoreLogic. More than 300,000 such loans will adjust this summer. CoreLogic's data cover about 80 percent of the mortgage market. Subprime refers to borrowers with poor credit histories.

Lenders, federal officials and housing counselors have worried that borrowers would not be able to afford the higher payments after the reset and would quickly fall into foreclosure. Declining home prices have made it impossible for many of these homeowners to refinance.

It will not be clear for months how many will lose their homes, Fleming said. "A lot of those are resetting now," he said. "We may not see the impact in foreclosures until the middle of 2009."

The mortgage industry has trumpeted efforts to reach homeowners and put them in affordable mortgages. Members of Hope Now, an alliance of mortgage lenders and nonprofit groups, have agreed to contact homeowners 120 days before their loans reset. This summer will test those efforts.

"There has been a lot of work done to meet the demand," said Faith Schwartz, head of Hope Now.

Since January, the alliance has been monitoring 718,000 subprime loans with resets scheduled for this year, looking for trends and problems, Schwartz said. Less than 1 percent of homeowners who paid their mortgage on time before their loans reset have fallen into foreclosure, she said. Others have sold their home or found a way to refinance, she said.

"We're trying to measure the effectiveness of the guidance" the industry adopted to combat the problem, Schwartz said. If the figures demonstrate problems among a subset of borrowers, the industry can react quickly, she said.

The threat of soaring adjustable-rate loans has been tempered by the Federal Reserve's interest-rate cuts over the last year, Schwartz said. A borrower with a typical-size subprime ARM of about $200,000 could expect payments to increase about $70 a month if it reset now. That compares with about $450 a month if it had reset in December, according to the American Securitization Forum, a financial-industry group.

Even so, analysts and housing counselors have noted that while the interest-rate decreases have helped some borrowers, others still face a rate shock because they had an artificially low introductory rate, known as a teaser rate, or other types of loans.

After reaching its peak this summer, the number of subprime adjustable-rate loans facing resets drops off significantly early next year, according to CoreLogic's figures. By January, only 4.8 percent of subprime loans will face resets. By May, that will fall to 1.94 percent.