Summary: The five Indian Health Service area offices GAO visited--Aberdeen, Alaska, California, Navajo, and Portland--differed greatly in the way that they delivered health care services. Nonetheless, the areas reported generally similar levels in the availability of basic clinical services. The services most available were treatment services, such as routine prenatal care, and diagnostic services, such as biopsies for cancer diagnoses. Almost all patients seeking such services were able to receive them. Preventive care, such as diabetes education and dental care, was comparatively less available. Service unit officials generally named alcohol and substance abuse services as their greatest unmet health need. Despite recent increases in Indian Health Service funding for alcohol and substance abuse treatment services, the gap between the demand for and availability of services persists. In addition, the Indian Health Service lacks data on alcoholism rates among native Americans and the effectiveness of current prevention and treatment programs.