Kaiser Permanente says a computer drive containing thousands of patient records was stolen from an employee's car.
Hospital officials said Tuesday the external drive stolen Dec. 1, 2009, held data on as many as 15,500 patients throughout Northern California, including the patient's name and Kaiser medical records number.
The records could have also included a patient's sex, birth date, phone number and other medical information.
Hospital spokeswoman Kristin Chambers says the theft presents a "low risk to our patients."
The hospital has fired the employee, who was authorized to access the data as part of her work, and says it is unclear why she took the device home.
