Citigroup lost almost $10 billion in last year's final three months, the largest quarterly deficit in the bank's 196-year history. As of Dec. 31, it had a total of $37.3 billion in direct subprime mortgage exposure, down from $54.6 billion three months prior. 

The nation's largest bank wrote down the value of its portfolio by $18.1 billion. It also boosted loan-loss reserves by $4.1 billion, signaling further problems in its consumer businesses as deflated home prices, high energy and food costs, and rising unemployment weigh on people's ability to make their loan payments.