To study prognostic factors 38 consecutive patients with aplastic anemia were evaluated until death or over a follow-up period of at least 4 years. A prognostic formula was devised by multivariate analysis for prediction of short-term survivors at diagnosis of aplastic anemia: Using the initial percentage of nonmyeloid cells in the marrow aspirate and the neutrophil count in peripheral blood, survival of less than 4 months is predicted with a probability of 89% (sensitivity 80%). The lowest blood counts during the course of the disease are useful to predict the outcome in more chronic courses of aplastic anemia: A reticulocytopenia below 5 X 10(9)/1, a neutropenia below 0.1 X 10(9)/1 or a thrombocytopenia below 5 X 10(9)/1 all resulted in an ultimate mortality of over 90%. However, if none of these threshold values were reached, a 4 year survival of 71% was observed. Thus, 5 X 10(9) reticulocytes/1, 0.1 X 10(9) neutrophils/1 and 5 X 10(9) platelets/1 form a group of risk factors to predict a fatal or favorable outcome throughout the course of the disease.