The bone marrow adipocyte colony-forming capacity (Adipo-CFC) in patients with aplastic anemia (AA), myelodysplastic syndrome (MDS) and iron deficiency anemia (IDA) was studied. Before treatment, Adipo-CFC in IDA was higher than that in AA and MDS. After treatment, Adipo-CFC decreased in IDA, but it increased in AA and MDS only at the responsive stage. In this context, it is suggested that increase of Adipo-CFC occurs during not only regenerating hemopoiesis but also accelerated erythropoiesis such as severe IDA. Colony-stimulating activity (CSA) production by marrow stromal cells (MSC) in AA was lower than that in normal subjects. Low Adipo-CFC and defective CSA production by MSC may explain in part the pathogenesis of microenvironmental defect in AA.