The telco said a laptop containing the personal details of 16,500 current and former employees was stolen from the car of an MCI financial analyst. The car is thought to have been left unlocked in an unlocked garage at the analyst's home.
The laptop is said to have been password-protected but it is not clear whether the personal data was encrypted.
The data, which went missing last month, included names and social security numbers of staff. The MCI financial analyst was using the data to analyse financial trends in the company.
MCI has sent out notification letters to employees whose personal information was on the stolen laptop. So far, the company says it does not have any evidence that the data has been used illegally.
Time Warner recently lost back-up tapes containing data on around 600,000 current and former employees, and ChoicePoint and the LexisNexis group have also recently seen personal data on hundreds of thousands of people go missing, as a result of ID thieves getting access to their databases.