"We buy at the top of the payment waterfall - triple-A and single-A - because we don't trust these structures enough to go lower." The number of downgrades witnessed, particularly from the 1997-1998 vintage, has never been higher. For example, 33% of Moody's Aa2-rated CDOs and 39% of the Baa3-rated CDO from the 1997-1998 vintage have been downgraded. Moody's attributes the severe downgrade pressure to a simplistic "common theme that CDOs that closed during that period paid dearly for their assets." Meanwhile, high event risk pressures in the corporate credit market triggered downgrades on 26 tranches of balance-sheet CDOs - half of all downgraded tranches in 2001. While having a deal on review or downgraded by the rating agencies makes marketing new CDO issues more difficult for those who have had problems and didn't work them out successfully, this hasn't discouraged some asset managers from returning to the market.