We treated myelodysplastic syndrome patients (MDS) with both recombinant human granulocyte colony-stimulating factor (G-CSF) and recombinant human erythropoietin (EPO) to determine whether such combination therapy resulted in improvement of their anemias. Twenty- four of 28 patients begun on study completed the protocol and were evaluable for erythroid responses. Therapy was initiated with G-CSF at 1 micrograms/kg administered by daily subcutaneous injection and adjusted to either normalize or double the neutrophil count. EPO was then administered by daily subcutaneous injection at a dose of 100 U/kg and dose-escalated to 150 and 300 U/kg every 4 weeks while continuing the G-CSF. Changes in absolute reticulocyte count, hematocrit level, and need for RBC transfusions were compared with pretreatment values as well as other blood cell counts. Ten of 24 patients (42%) had erythroid responses, whereas all patients had neutrophil responses. Six previously transfused patients no longer required RBC transfusions during the treatment period. Erythroid responses were found to be independent of patient age, French-American-British subtype, duration of disease, prior RBC transfusion requirements, or cytogenetic abnormalities at presentation. Pretreatment serum EPO levels were lower in erythroid-responding as compared with nonresponding patients (median 157 v 600 U/L; P = .05). The combined treatment modality was generally well tolerated. We conclude that a substantial percentage of MDS patients had both erythroid and myeloid responses when treated with the combination of G-CSF and EPO.