Two groups of patients with a myelodysplastic syndrome (MDS) were analyzed by univariate (log-rank test) and multivariate (logistic regression) analyses to detect the most important prognostic factors. By stepwise analysis, the variables found to have prognostic significance for death were as follows: age, percentage of marrow blasts, presence of circulating blasts, and number of platelets. The variables found significant for predicting progression to acute leukemia (AL) were as follows: hemoglobin level, percentage of marrow blasts, and presence of circulating blasts. The first group of 193 patients was used to build a prognostic index which reflected the probability of a given patient dying or progressing to AL within 6, 9, or 12 months. The application of this prognostic index to a test group of 143 patients was used to determine the expected error rate and the validity of the prediction rule.