Acute leukemia of myeloblastic or erythroblastic morphology occasionally occurs as a complication of idiopathic refractory sideroblastic anemia, but the development of acute lymphoblastic leukemia has not been previously reported in these cases. A patient with idiopathic refractory sideroblastic anemia is described in whom acute lymphoblastic leukemia occurred. The leukemic cells were characterized by typical lymphoblastic morphology on Wright's stain, periodic acid-Schiff-positive cytoplasmic clumps, elevated levels of deoxynucleotidyl transferase (143 units/10(8) cells), high numbers of specific glucocorticoid binding sites (16,845 sites/cell, Kd = 5.40 x 10(-9) M), non-B, non-T cell immunologic characteristics and clinical responsiveness to therapy with vincristine, prednisone, and methotrexate. Ultrastructural studies of the lymphoblasts identified ferruginous material in lysosomes and occasional mitochondria similar to but less abundant than that seen in abnormal sideroblasts. The concurrence of these two disorders supports the theory that in humans both lymphoid and myeloid cell lines arise from a common pluripotent stem cell.