TD Bank has lost data tapes containing the names, account information and social security numbers of 260,000 US customers.
The unencrypted tapes were lost during transit in March, a spokeswoman told Reuters, stressing that there is no evidence that the data has been misused.
TDF Bank says it is investigating the loss, has called in law enforcement and will offer affected customers credit monitoring and identity theft protection.
In a statement, Martha Coakley, Attorney General for Massachusetts - home of 73,000 of the customers - says: "We will be reviewing the circumstances of this breach and the steps that TD Bank is taking to address the loss."