Erythropoietin treatment is known to correct anemia and to improve hemostasis. Since platelets may contribute to thromboembolic complications, we assessed platelet aggregation in whole blood and platelet-rich plasma from chronically hemodialyzed patients treated with erythropoietin and evaluated in vitro effects of this drug on aggregatory responses of uremic and normal platelets. Recombinant human erythropoietin was given to uremic patients at a dose of 2,000 IU subcutaneously three times a week. Platelet aggregation in whole blood and platelet-rich plasma was induced by collagen, ADP, arachidonic acid, and ristocetin. In uremic patients, erythropoietin therapy resulted in an enhancement of platelet sensitivity to various agonists, particularly in platelet-rich plasma, reaching values comparable to those of healthy volunteers. In vitro studies we were unable to show any direct effect of erythropoietin, used at concentrations that occurred post intravenous administration, on platelet aggregation both in whole blood and in platelet-rich plasma.