We analyzed the prognostic value of clinical, hematologic and bone marrow (BM) histologic findings at presentation in 94 patients with myelodysplastic syndromes (MDS) (28 RA; 2 RARS; 34 RAEB; 6 CMML; 24 RAEB-t). With survival as the dependent variable, stepwise multivariate analysis indicated as the prognostically most important factors among the MDS taken as a whole: latency from the first symptoms to diagnosis, age, and percentage of BM blasts. In each main MDS group the most unfavorable initial characteristics were: 1) low Hb, no macro-megaloblastosis, male sex for RA/RARS; 2) low Hb and low platelet levels for RAEB/CMML; 3) granuloblastic hyperplasia and high BM blastosis for RAEB-t. Of the BM histologic parameters, only the percentage of blasts had significant prognostic value. Histologic assessment of BM blastosis, however, did not differ statistically from that based on cytologic examination of BM smears, so that marrow histology seemed not essential for initial prognostic assessment in MDS patients. The finding of abnormal localization of immature precursors (ALIP) in BM biopsies was associated with a negative trend without reaching statistical significance. Using four objective parameters of proven significance (age, Hb, platelets, and BM blasts) we devised a staging system of immediate clinical utility for prognostic stratification and risk-adapted therapeutic choices.