Myelodysplastic (MDS) patients at diagnosis (n = 162) were analysed by the International Prognostic Scoring System (IPSS). The two intermediate groups were not significantly different. The IPSS was of limited value in predicting survival of MDS patients after preliminary separation into subgroups with < 5%, or >/= 5%, myeloblasts. Cox's proportional hazards analysis of these subgroups enabled discrimination of highly significant prognostic groups. In both subgroups, longer survival was associated with macrocytosis. Mean corpuscular volume (MCV) and marrow myeloblast number were used to define four groups with prognostic significance similar to the IPSS. A low-risk group was described by macrocytosis associated with < 5% myeloblasts and high risk was related to blast counts >/= 5% and MCV < 100 fl. Further analysis defined factors identifying very high-risk patients and those with benign disease, together with many intermediate survival patterns. Results were consistent in two time-separated patient groups.