J.P. Morgan Chase & Co., the nation's largest bank, surprised the market today, saying it has taken large losses stemming from derivatives bets gone wrong in the bank's Chief Investment Office.

At 4:30, the bank sent out an unusual notice saying that it would be holding a call at 5 p.m. but included no details about what the call would be about. A person familiar with the matter said the call would include CEO Jamie Dimon and discuss the bank's quarterly filing.

On...