We analyzed the natural history of idiopathic refractory sideroblastic anemia (IRSA) in 37 patients studied between 1969 and 1986. Although erythroid abnormalities were prominent in all, 12 patients also showed involvement of the granulocytic and/or megakaryocytic cell lines, and nonrandom chromosomal aberrations were observed in five of 23 patients studied for such defects. Measurements of erythroid marrow function showed in most cases erythroid expansion with ineffective erythropoiesis. In seven patients, however, the erythroid activity was found to be inappropriately low for the degree of anemia. Transfusion dependence occurred in 26 of 37 cases. Iron overload was a common feature at presentation but produced clinical manifestations of hemochromatosis only in those patients who subsequently had a regular need for blood transfusions. Five patients progressed to bone marrow failure, and another five patients (two of whom had monosomy 7) evolved into acute nonlymphocytic leukemia (ANLL). The median survival was 72 months, with a high transfusion requirement, multilineage defects, and inappropriately low erythroid proliferation being associated with a poor prognosis. The most common causes of death were complications of iron overload and evolution into ANLL. We conclude that (a) the natural history of IRSA is characterized by an initial phase of erythroid hyperplasia and ineffective erythropoiesis, which is usually stable for many years but in a subset of patients may be followed by a phase of marrow failure with or without the later emergence of leukemic blasts; (b) peripheral blood counts, measurement of erythroid marrow function, and chromosomal analysis are useful for identifying subjects at risk of evolution into marrow failure or ANLL; and (c) IRSA patients with no need for blood transfusions are very likely to be long survivors, whereas those who become transfusion dependent are at risk of death from the complications of secondary hemochromatosis.