In his annual Berkshire Hathaway letter, Warren E. Buffett recently urged investors to pose tough questions at the shareholders meeting in May. Here is one on the mind of some Buffett watchers: When are you going to fix Moody’s?

Mr. Buffett, known as the Oracle of Omaha, owns a stake of roughly 20 percent in the Moody’s Corporation, parent of one of the three rating agencies that grade debt issued by corporations and banks looking to raise money. In recent months, Moody’s Investors Service and its rivals, Standard & Poor’s and Fitch Ratings, have been prominent in virtually every account of the What Went Wrong horror story that is the financial crisis.

The agencies put their seals of approval on countless subprime mortgage-related securities now commonly described as toxic. The problem, critics contend, is that the agencies were paid by the corporations whose debt they were rating, earning billions in fees and giving the agencies a financial incentive to slap high marks on securities that did not deserve them.

At least 10 of the big companies that failed or were bailed out in the last year had investment-grade ratings when they went belly up  like deathly ill patients bearing clean bills of health.