The tapes contained names, Social Security numbers, account numbers and payment history of customers of CitiFinancial, a division of Citigroup that provides consumer loans and services.
The tapes were in a box that was lost in a routine shipment to a credit bureau, the company told affected customers. There has been no indication that the data has been used inappropriately, according to CitiFinancial.
"We deeply regret this incident, which occurred in spite of the enhanced security procedures we require of our couriers," Kevin Kessinger, executive vice president of Citigroup's global consumer group and president of consumer finance, North America, said in a statement.
Beginning next month, the customer data will be sent direct to credit bureaus via encrypted electronic transmission, he said.
There is little risk of affected customers' accounts being compromised because they have already received their loans and no more credit can be obtained from the company without customer approval, he said.
The missing tapes contained information on CitiFinancial branch network customers in the U.S. and customers with closed accounts from CitiFinancial Retail Services, but no customer data from CitiFinancial Auto, CitiFinancial Mortgage or any other Citigroup business, the company said.
The incident is just the latest breach exposing sensitive customer information. Last month, Time Warner said a storage partner lost a batch of backup tapes containing personal information of 600,000 current and former Time employees.