Two patients with idiopathic refractory sideroblastic anemia (IRSA) were found to have acquired hemoglobin H. Both developed leukemic syndromes. Six other patients with IRSA had normal hemoglobin electrpohoretic patterns, and none has developed acute leukemia in up to five years. We suggest that hemoglobin electrphoresis be performed routinely in IRSA. The finding of hemoglobin H in this population may be a relatively frequent occurrence and may serve to distinguish the preleukemic patients from those not destined to develop leukemia.