REUTERS / Samantha Sais
The information was stored on computers that were stolen from Sutherland Healthcare Solutions in Torrance, Calif., which provides billing services for the health departments of Los Angeles County and San Francisco.
The breach affects 55,900 patients in San Francisco and 168,500 patients in Los Angeles County.
Most of the affected patients are uninsured, according to a press release from the San Francisco Department of Public Health.
Personal information that was compromised includes names, billing information, some social security numbers, dates and locations of services, and dates of birth.
The theft underscores the dangerous possibility of data being stolen from contracted companies that have been entrusted with thousands of people's sensitive personal information.
Los Angeles County Assistant Auditor-Controller Robert Campbell told the Los Angeles Times that he's "not aware of another breach of this significance ever having occurred."
The health departments are notifying those affected by mail, but it might be challenging to reach everyone because some patients might have changed addresses or have difficulty receiving mail, said Rachael Kagan, a spokeswoman for the San Francisco Department of Public Health. San Francisco is also organizing other outreach efforts to track down affected patients.
Free credit monitoring will be offered to those whose information might have been compromised.