The appearance of sideroblasts was studied in 277 patients with various blood disorders. The appearance rate of sideroblasts was 84.1 ± 11.8% in hypoplastic anemia and 70.7 ± 23.7% in acute myelocytic leukemia, whereas it was 29.5 ± 7.8% in healthy controls. The standard sideroblastogram pattern in patients with hypoplastic anemia was type Ⅱ sideroblast dominance with moderate sideroblastosis. Three out of 87 cases of hypoplastic anemia showed type Ⅲ sideroblast dominance with conspicuous sideroblastosis. Their hematological findings were those of atypical hypoplastic anemia. One of them terminated in acute myelocytic leukemia and could be considered hypoplastic preleukemic stage retrospectively. On the other hand; the sideroblastograms of 6 out of 41 cases of acute myelocytic leukemia showed type Ⅲ sideroblast dominance. Four of them were identical with low percentage leukemia. One of these 4 cases showed ringed sideroblasts on admission and terminated in erythroleukemia at the last stage. These results suggest that in hypoplastic anemia type Ⅲ sideroblast dominance should be considered to be atypical hypoplastic anemia or hypoplastic preleukemic stage, which indicates distrurbed erythroid maturation of preleukemia.