Two legal organizations say health insurer Aetna revealed the HIV status of patients in several states by mailing envelopes with a large, clear window that showed information on purchasing HIV prescriptions.
The Legal Action Center and the AIDS Law Project of Pennsylvania claim some patients' relatives and neighbors learned of their HIV status as a result.
Aetna said that "this type of mistake is unacceptable" and that the company is reviewing processes to ensure it never happens again.
The Hartford, Connecticut-based Aetna started notifying customers of the breach in letters sent this week.
The legal organizations sent a cease-and-desist letter to Aetna. They want the insurer to show what corrective measures are being taken.
Patients were in Arizona, California, Georgia, Illinois, New Jersey, New York, Ohio, Pennsylvania and Washington, D.C.