Patients are now being notified about the security breech.
Police tell ABC7 the records were on a portable hard drive and stolen from the Westmont office of Millennium Medical Management Resources.
It happened back in February.
The company handles billing for emergency healthcare physicians. Letters are being sent to EHP patients indicate people who were treated between 2003 and 2006 may be affected by the theft.
ABC7 was unable to get any comment from either Millennium Medical Management Resources or Emergency Healthcare Physicians.