JPMorgan Chase Bank has said a data breach has affected 76 million households and seven million small businesses.
The New York-based bank says there is no evidence that the account information for the affected customers, such as account numbers, passwords or social security numbers, was stolen.
The bank says it has not seen any unusual customer fraud related to the hack.
JPMorgan Chase says a recent cyber attack compromised customer information.
The nation’s biggest bank by assets, it has been working with law enforcement officials to investigate the cyber attack.
Jamie Dimon, the bank’s chief executive, said in this year’s annual report to shareholders that despite spending millions on cyber security, JPMorgan remained worried about the threat of attacks.
By the end of this year, the bank estimates that it will be spending about $250m (€197.29m) annually on cyber security and employing 1,000 people in the area.
In August, the FBI said that it was working with the Secret Service to determine the scope of recent cyber attacks against several American financial institutions.