Woo ... it looks like the housing market slowdown and default nightmare for mortgage underwriters will be worse before getting better. If these Alt-A customers are turning in 2.4% delinquency rate, there is no wonder why my person-to-person lending experience at Prosper.com is abysmal.

At issue are mortgages made to people who fall in the gray area between "prime" (borrowers considered the best credit risks) and "subprime" (borrowers considered the greatest credit risks). A record $400 billion of these midlevel loans -- which are known in the industry as "Alt-A" mortgages -- were originated last year, up from $85 billion in 2003, according to Inside Mortgage Finance, a trade publication. Alt-A loans accounted for roughly 16% of mortgage originations last year and subprime loans an additional 24%.

The catch-all Alt-A category includes many of the innovative products that helped fuel the housing boom, such as mortgages that carry little, if any, documentation of income or assets, and so-called option adjustable-rate mortgages, which give borrowers multiple payment choices but can lead to a rising loan balance. Loans taken by investors buying homes they don't plan to occupy themselves can also fall into the Alt-A category.

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Data from UBS AG show that the default rate for Alt-A mortgages has doubled in the past 14 months. "The credit deterioration has been almost parallel to what's been happening in the subprime market," says UBS mortgage analyst David Liu. The UBS report contrasts with testimony Federal Reserve Board Chairman Ben Bernanke gave to Congress yesterday. "Our assessment is that there's not much indication that subprime issues have spread into the broader mortgage market," Mr. Bernanke said.

To be sure, defaults have remained very low in the prime market -- and despite the uptick in bad loans, the problems in the Alt-A sector aren't as severe as those that have roiled the subprime market. Some 2.4% of Alt-A loans are at least 60 days past due, according to UBS, which looked at mortgages that were packaged into securities and sold to investors. That is well below the 10.5% delinquency rate for subprime mortgages. (During the housing boom, delinquencies were low for all types of loans because borrowers who wound up in trouble could refinance or sell.)