On cytological bone marrow examination we distinguished between pure sideroblastic anaemia (PSA), which is confined to dyserythropoiesis, and refractory anaemia with ring sideroblasts (RARS), which is characterized by additional dysplastic features of granulopoiesis and/or megakaryopoiesis. In a follow-up study of 94 patients with AISA diagnosed according to FAB criteria for myelodysplastic syndromes we found a striking difference in the risk of leukaemic transformation between PSA and RARS (5 year cumulative rate 1.9% v. 48%). Overall survival was much better in PSA than in RARS (5 year cumulative chance 69% v. 19%). Infections and haemorrhages were frequent causes of death in RARS but not in PSA. Bone marrow culture studies (CFU-GM) were performed on 10 consecutive patients with PSA and RARS, respectively. RARS patients showed grossly impaired colony growth, typical of the myelodysplastic syndromes. Patients with PSA had persisting colony formation, even if moderately decreased in frequency, with numbers of CFU-GM being inversely correlated with the degree of erythroid hyperplasia in the bone marrow. We conclude that on cytomorphological grounds AISA can be divided into pure (dyserythropoietic) sideroblastic anaemia (PSA) and a true myelodysplastic form (RARS), with both types differing considerably in terms of survival, risk of leukaemic transformation and findings on bone marrow culture (CFU-GM).