Hematologic and cytogenetic data were collected on 401 myelodysplastic syndromes (MDS) patients in Japan and their clinical relevance was analyzed. More than 50% of the MDS patients with hypocellular bone marrow had > 5% marrow blasts at the time of MDS diagnosis and frequently had complex aberrations (chromosome changes at three or more regions). They showed peripheral blood findings resembling those of aplastic anemia, but progression into leukemic phase and the prognosis tended to mimic typical MDS. In MDS patients with minimal dysplasia, hematologic parameters were different from those of aplastic anemia. However, the low incidence of leukemic transformation and the favorable prognosis was similar to that of aplastic anemia. More than 90% of the patients in this group had refractory anemia and had normal karyotypes. Thus differential diagnosis from a low grade aplastic anemia is important, since this type of MDS might have a clonal nature as well. In conclusion, although some hematologic and clinical deviations are noticed in MDS with hypocellular marrow or minimal dysplasia, these MDS subtypes might constitute marginal forms of MDS.