Sutter Medical Foundation officials say a computer stolen last month contains personal information on more than 4 million patients dating back to 1995.
The Sacramento Bee reports the database includes the names, addresses, email addresses, dates of birth, telephone numbers and names of the health insurance plans of 3.3 million patients.
Information on another 943,000 patients also includes medical record numbers, dates of services and a description of medical diagnoses and procedures.
The Sacramento-based physicians' network says the data did not include patient financial records, social security numbers, patients' health plan identification numbers or medical records.
Affected patients are being notified by mail if their information was compromised. Sutter says it's speeding up efforts to encrypt all its computers but that the stolen desktop computer's data was not encrypted.