To define a survival-based severity measure for AIDS, we convened a panel of AIDS experts who identified factors influencing AIDS prognosis and estimated the impact of prognostic factors on survival time. The resulting conceptual model included 19 AIDS-defining conditions and 80 subsequent AIDS-related complications. This model was tested on longitudinal disease histories of 3,937 AIDS patients in the New York State Medicaid Program diagnosed between 1983 and 1986 and followed through 1988. The initial AIDS diagnosis and complications within 3 months of AIDS onset were identified from coded diagnoses recorded on inpatient and outpatient claims. Three AIDS-defining diagnosis groups were created; survival times from least to most severe group were 25, 10, and 7 months. To determine the influence of subsequent complications on risk of death, the survival times associated with combinations of defining diagnosis groups and four severity levels of subsequent complications were determined. Median survival ranged from 43 months for the least severe defining diagnosis group without early complications to 12 months for the group with severe defining diagnoses and serious complications. The matrix of AIDS-defining diagnoses and complications was divided into four severity categories with significantly different survival curves. This severity measure uses longitudinal data commonly available to clinicians and researchers to create distinct AIDS prognostic categories.