FLOW CYTOMETRY WAS used to determine the deoxyribonucleic acid ploidy and proliferative activity of 230 astrocytomas. The relationships among survival, ploidy, proliferation, histological features, and clinical variables were analyzed. Multivariate analysis confirmed the independent prognostic significance of the S-phase fraction (P< 0.01), ploidy (P= 0.04), age at diagnosis (P< 0.001), extent of surgery (P< 0.01), and sex (P= 0.03). Three groups with significantly different survival were defined based on S-phase fraction ranges of < 3%, 3%-5.9%, and ≥ 6%. The strong correlation between the S-phase fraction and survival confirmed the importance of quantitative proliferation assays in predicting tumor behavior and demonstrated that specific reference ranges can be defined for clinical application. The weaker association between ploidy and survival leaves the usefulness of the determination of ploidy with flow cytometry in doubt.