Twenty-five consecutive patients with myelodysplastic syndrome (MDS) were followed in the Second Department of Internal Medicine, Miyazaki Medical School from 1984 to 1993. The diagnosis of MDS was morphologically based on the criteria of FAB. At the time of diagnosis, 9 patients had refractory anemia (RA), 1 had RA with ring sideroblasts (RARS), 6 had RA with excess blasts (RAEB), 6 had RAEB in transformation (RAEB-t), and 3 had chronic myelomonocytic leukemia (CMMoL). Prognostic factors involved in survival times and progression to leukemia were analyzed in these patients; FAB classification of MDS, age, sex, peripheral blood cell counts, bone marrow examination, karyotype, numbers of blasts. None of these prognostic factors had a significant effect on the prognosis of MDS patients. Study of the therapeutic effects on MDS patients revealed no significant increase of survival time in treated MDS patients compared to non-treated patients. Further, no significant difference in survival time was found between MDS patients treated with or without anticancer drugs. These results indicated that MDS patients were pathologically and therapeutically heterogeneous.