To assess the incidence and the eventual prognostic relevance of pathologic sideroblastosis in myelodysplastic syndromes and acute nonlymphocytic leukemia, 5 acquired idiopathic sideroblastic anemias, 2 refractory anemias, 9 refractory anemias with excess of blasts, 10 acute nonlymphocytic leukemias were studied with regard to sideroblast type (ring or ferritin) and percentage. Pathologic sideroblastosis was commonly found in each subgroup, in some patients it appeared later in the course of the disease while in other reversed. In a successfully treated leukemic patient, pathologic sideroblastosis in otherwise normal bone marrow after therapeutic hypoplasia suggested clonal remission. Although the patient number was small, pathologic sideroblastosis seems to correlate with poor treatment response in leukemic patients. We conclude that systematic looking for pathologic sideroblastosis may have some biologic and clinical implications in myelodysplasia and acute leukemia.