Background: Trends in the care of patients with cancer are monitored annually by the Commission on Cancer of the American College of Surgeons. In 1991 a patient care evaluation study of breast cancer was conducted, which among other questions examined the correlation of health insurance with type or quality of care delivered for breast cancer on a national basis. Methods: The tumor registry system of the American College of Surgeons was used to obtain data on patients with breast cancer diagnosed in 1983 and 1990. Trends in diagnosis and treatment were correlated with the type of insurance or lack of insurance. Results: Data were obtained from hospitals in 50 states on a total of 41,651 patients. The largest number of patients were covered by Medicare. Fewer than 5% were considered medically indigent. Medically indigent patients presented with higher stage disease and did not participate in a trend toward downstaging, which occurred between the two study years. The treatment of medically indigent patients appeared to be appropriate and comparable with better insured patients. Insurance type (health maintenance organization vs. private) did not affect stage, treatment, or outcome. Decisions to use controversial therapies, such as chemotherapy for stage I disease, did not appear to be financially driven. Conclusion: A nationwide pattern of care study for breast cancer indicates that medically indigent patients present with more advanced disease compared with better insured patients, but once the diagnosis is made, treatment and outcome have little to do with insurance type. © 1994 The Society of Surgical Oncology, Inc.