Treatment with recombinant human erythropoietin (rhEPO) for the anemia of end-stage renal disease has been associated with thrombotic complications. To detect prothrombotic changes in autologous blood donors given 500 U/kg rhEPO subcutaneously (twice weekly during a 3-week period), changes in variables of hemostasis and fibrinolysis and in blood rheology before and at the end of treatment were investigated. In 21 patients, platelet count increased from 272 +/- 55 x 10(9)/L to 313 +/- 55 x 10(9)/L (p < 0.05). Although activated partial thromboplastin time and protein C antigen decreased significantly during rhEPO treatment, these changes remained within normal ranges. No changes in the hemostatic variables prothrombin time, fibrinogen, factor V, von Willebrand factor antigen, antithrombin III activity, protein S antigen, and prothrombin fragments F 1 + 2 were found. Measurements of plasminogen activity, alpha 2-antiplasmin activity, tissue plasminogen activator, and plasminogen activator inhibitor-1, representing variables of fibrinolysis, were normal and constant during the study. In 5 patients no changes in red cell deformability and whole blood viscosity, corrected for differences in hematocrit, were observed. Plasma viscosity showed a slight but clinically not relevant increase in 4 out of 5 patients. The absence of evident (pro)thrombotc changes in this study confirms the safety of high-dose rhEPO therapy in autologous blood donors, who donate 2 units (i.e., 2 x 450 ml) of blood.