The Observational Data Base (ODB) is a multicenter, longitudinal effort of the Community Programs for Clinical Research in AIDS (CPCRA) designed to collect HIV-related data on a large number of HIV-infected patients receiving primary care from community based physicians and health care groups. Over 400 patients from the Addiction Research and Treatment Corporation have enrolled in the ODB. Compared to the remainder of the CPCRA, ARTC ODB enrollees are more likely to be female, African American or Hispanic and to have injectable drug use and heterosexual contact with an injectable drug user as risk factors. The ARTC ODB patient profile closely resembles the fastest growing segments of the AIDS epidemic.