To identify prognostic factors in acquired immunodeficiency syndrome (AIDS), the authors studied an inception cohort of 45 patients in a non-endemic area (Group I). The probability of survival was 67% six months after the diagnosis of AIDS and 32% at 12 months. As shown by multivariate Cox regression analysis, survivals were shorter (p<0.01) in patients 35 years old or older and in those who had anemia when AIDS was diagnosed. In patients with neither of these poor prognostic factors, the 12-month survival was 64%; in patients with one factor, it was 22%; and in patients with both factors, 0%. The prognostic significance of these two factors was validated in a second inception cohort of 50 patients (Group II): in patients with zero, one, and two poor prognostic factors, the 12-month survivals were 80%, 58%, and 26%, respectively. Other poor prognostic factors in Group I included disseminated Mycobacterium avium-intracellulare and the development of new opportunistic infections or neoplasms. The authors conclude that clinically important prognostic factors can be identified in AIDS patients. These findings should be considered in planning therapeutic trials and in counseling patients. © 1988 the Society of General Internal Medicine.