The prognostic significance of the bone marrow sideroblasts found in aregenerative anaemia with hypercellular bone marrow was evaluated particularly to see if the sideroblasts might be a preleukaemic sign. The study includes 32 patients with abnormal sideroblasts; 14 had mainly ringed sideroblasts while 18 had mainly the intermediate or ferritin type. Basic erythrokinetic data (59Fe erythrocyte incorporation and erythroblast labelling indices) were similar in the two groups. The colony forming capacity (CFUc) was abnormal in 2 of 5 patients with ringed sideroblasts compared to 12 of 13 in the intermediate sideroblast group. Patients with mainly ringed sideroblasts had significantly longer duration of disease state, higher platelet and leucocyte counts, no increase in the percentage of bone marrow myeloblasts and did less often develop acute leukaemia. Thus aregenerative anaemias can be divided according to the type of abnormal sideroblasts in two groups with different risk of development of acute leukaemia. The ringed sideroblastic anaemia seems not to be a preleukaemic stage in the prospective sense in contrast to aregenerative anaemia with mainly intermediate sideroblasts.