The appearance of sideroblasts was investigated before and after cancer chemotherapies which were carried out 65 times in 53 patients with malignant neoplasms. After treatment, 34 cases showed marked sideroblastosis with a shift to the right in their sideroblastograms ("shifted group"). The other 31 cases showed no sideroblastosis without a shift in their sideroblastograms ("non-shifted group"). The hematological data from these two groups were then studied. After chemotherapy, the shifted group showed significant decreases in erythrocyte counts, hemoglobin concentration, leukocyte counts and nucleated cell counts and a significant increase in erythroid/myeloid ratio, whereas the non-shifted group showed no significant changes. Furthermore, the incidences of serious anemia, leukopenia, decrease in nucleated cell counts and remarkable elevation in serum iron level were significantly higher in the shifted group than in the non-shifted group. These findings suggest that marked sideroblastosis is a useful indicator in the evaluation of impaired hematopoiesis due to cancer chemotherapy.