The Detroit Medical Center says an employee stole the personal information of 1,087 patients at Harper University Hospital.
The health care company says Harper sent letters to the affected patients after West Bloomfield police informed the hospital of the theft.
Police found the personal documents while they were investigating an identity theft.
Authorities say the documents include names, dates of birth, reasons for patients' visits and in some cases Social Security numbers.
The hospital says it's offering free credit monitoring to the affected patients.