The number of homeowners facing an increase in their subprime adjustable-rate mortgage payments will peak this summer, testing the efforts of lenders and others to keep those people out of foreclosure and stabilize the housing market.

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 (2/28 and 3/27 loan products). For many, an adjustment means their interest rate will go up two to three percentage points, based on the index the loan was tied to.

Nationally, the number of subprime adjustable-rate loans resetting peaked at 7.61 percent of the loans outstanding last month, according to data from CoreLogic. More than 300,000 such loans will adjust this summer. CoreLogic's data covers about 80 percent of the national mortgage market.

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

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 of 2009, only 4.8 percent of subprime loans will face resets, compared with 7.61 percent this month. By next May, that will fall to 1.94 percent