Hematological disorders are commonly complicated by anemia, and the symptoms of red cell deficiency adversely affect the quality of life. Erythropoietin is a glycoprotein which controls red blood cell production. Recombinant human erythropoietin, 50 U/kg/day, was given subcutaneously to 16 patients with myelodysplastic syndrome and anemia. All but one patient was transfusion dependent. Diverse pretreatment endogenous serum erythropoietin levels were noted and ranged from 17 to 3616 IU/l. Two patients (12.5%) demonstrated an improvement in hemoglobin levels obviating the need for transfusions. Their responses lasted 5+ and 7 months with maintenance erythropoietin treatment. The responders had endogenous serum erythropoietin levels of 44 and 170, respectively. Treatment was generally tolerated without constitutional side-effects. However, three patients developed thrombocytopenia and one developed joint pain and leukocytosis on treatment. Overall, six patients showed changes in non-erythroid cells: two patients had an increase in platelet counts; three patients, a decrease in platelet counts; and one patient, an increase in white blood cell counts. Most of these changes reversed rapidly once erythropoietin was stopped. It is concluded that (a) serum erythropoietin levels are extremely variable in anemia patients with myelodysplastic syndrome, (b) only a minority of patients benefit from treatment with recombinant human erythropoietin, and (c) erythropoietin can affect cells of the myeloid and megakaryocytic lineage in a small proportion of patients.