The Johns Hopkins AIDS Service is the principal provider of medical care for HIV-infected patients in Maryland, a state in which the majority of HIV-infected patients live in an urban environment. A component of the HIV service at Johns Hopkins Medical Center is an information system that is used to track longitudinally the ambulatory and inpatient care of HIV-infected patients. Enrollment into this database coincides with first enrollment into the HIV Service. Extensive laboratory, diagnostic, clinical, and pharmaceutical information is collected at enrollment and is updated every 6 months. Outpatient and inpatient medical records, Johns Hopkins Health System automated databases, supplemental medical records from outside facilities, vital records, and patient and provider interviews are all used to obtain the detailed data that are stored on the database. The database also includes an economic component, which was added in 1994. This component links all Maryland state Medicaid claims data to that relating to patients who are insured by the Maryland Medical Assistance program, who account for approximately 60% of patients using the Johns Hopkins HIV Service. This data linkage facilitates detailed quantification of the costs of medical care for the HIV-infected patient throughout the course of the infection. We currently have data on about 3,000 HIV-infected patients representing a heterogeneous mix by race, sex, socioeconomic status, and risk factors for HIV transmission. Our data have been used to address a variety of issues regarding access to, utilization of, and clinical outcomes of HIV therapeutics. Clinical practice data such as ours will be increasingly important as the number and types of antiretroviral and other drugs for HIV infection continue to increase.