Ross-Loos Medical Group was a comprehensive prepaid health services plan and hospital located on West Temple Street in Los Angeles, California.

Ross-Loos was established in 1929 by two physicians, Donald E. Ross and H. Clifford Loos, older brother of writer Anita Loos. The plan of monthly payments assured benefits of socialized medical and hospital care to over two thousand employees of the Los Angeles County Department of Water and Power and their families.

The founders believed that health care could be improved by combining prepayment of services, eliminating the financial barriers faced by patients at the time of needed care, with the sharing of medical records and the ease of consultation in a medical group. The focus was on the improvement of medical care quality, rather than financial success, and many of the concepts in these plans built on a public health approach that encouraged prevention. These plans included prenatal care, well-baby visits, and immunizations in standard benefit packages, with small or no copayments, in an era when even the hospital costs of maternity stays were often excluded from traditional insurance.

Thus, Ross-Loos Medical Group was the first "health maintenance organization" (HMO) in the United States, a name that came into being in the 1970s.

Over several years many other employee groups, mostly governmental, joined the plan. Ross-Loos was so successful that the first small clinic expanded into an enormous organization. Besides clinics, the plan had nineteen offices and a large hospital by 1979.

In 1980, Ross-Loos Medical Group was purchased by the Insurance Company of North America (INA).