The effect of human recombinant erythropoietin (rhEPO) was investigated in 29 anemic patients with myelodysplastic syndromes (MDS). A rhEPO dosage of 150 U/kg was administered subcutaneously three times weekly for a minimum of 6 weeks. Seven out of 27 evaluable patients (26%) had an effective clinical response to therapy by increasing hemoglobin concentrations by more than 15 g/l (reaching at least 105 g/l) or by eliminating transfusion requirements. Six out of the seven patients responded within four weeks. Three of the responders successfully continued rhEPO treatment 15 months or more. To determine whether it may be possible to predict response to rhEPO, various clinical parameters were examined. Responders were found to be significantly different from non-responders in five aspects: They had less elevated baseline serum EPO levels (92 +/- 33 versus 515 +/- 108 U/l, mean +/- SEM; p = 0.023) and were more often transfusion-independent (71% versus 20% of non-responders; p = 0.022). Furthermore, responders were more often females (71% versus 40% in the non-responding group; p = 0.025), of subtype RA rather than RAEB (four patients and one patient, respectively, compared to seven and nine patients in the non-responding group; p = 0.025), and they predominantly displayed normal karyotypes or a 5q- aberration (86% versus 47%; p = 0.005). We conclude, that rhEPO treatment can reduce anemia in MDS and that certain pre-treatment clinical parameters may be used to predict response.