To define predictors of survival we studied longitudinal histories of 789 New York State Medicaid-enrolled children diagnosed with acquired immunodeficiency syndrome (AIDS) from 1983 to 1989 and followed through 1990. Median survival times for 3 severity groups of AIDS-defining conditions were 66, 48 and 9 months. In a proportional hazards model, the relative risk of death for the most vs. least severe group was 3.33 (95% confidence interval, 2.53, 4.37) and the relative risk for children < 6 months old at diagnosis vs. older children was 1.81 (95% confidence interval, 1.41, 2.34). We increased our ability to predict death by using a 4-category severity index that assesses both the AIDS-defining diagnosis and clinical complications within 3 months of diagnosis (relative risk, 5.27; 95% confidence interval, 3.16, 8.78 for most vs. least severe). These analyses offer new clinical severity measures and reveal that, regardless of the AIDS-defining diagnosis, children with AIDS who are < 6 months old have a poor prognosis.