The theft happened in Sacramento on December 1st -- but the HMO didn't go public with details until this week.
Data stored on the computer include patients' full names and medical numbers, and in some cases, birth dates, phone numbers and medical treatment information.
It did not contain social security numbers or credit card numbers. Kaiser has set up a phone line for people who want more details.
People with questions about the information leak can call Kaiser at 877-608-0050. Patients covered by Medicare can call 800-443-0815.