Although the haemostatic defects that occur in uraemia are complex, a major factor is the anaemia of renal failure. This may now be corrected by recombinant human erythropoietin (rHuEpo) therapy rather than by repeated blood transfusion. Platelet reactivity to shear stress and collagen was measured using non-anticoagulated blood to study the effect of erythropoietin or blood transfusion on platelet function. Twenty dialysis patients were commenced on 25-50 U/kg rHuEpo twice weekly. The dose was adjusted after 3 months to maintain target Hb 10-10.5 g/dl. A further 15 patients were studied before and 10-12 days after receiving blood transfusion. Baseline platelet reactivity was subnormal in both groups versus control (P < 0.0001). In the rHuEpo group, a significant increase in platelet reactivity was observed at 2 months (P < 0.005) which disappeared at 3 months. This was not related to the increase in Hb (7.3 +/- 0.3 to 10.2 +/- 0.3 g/dl, P < 0.0001). There was no change in platelet reactivity after transfusion, despite an increase in Hb (6.2 +/- 0.2 to 8.8 +/- 0.2 g/dl, P < 0.0001) similar to that occurring in the rHuEpo group. We conclude that after rHuEpo therapy but not after transfusion a transient increase in platelet reactivity occurs which is dissociated from changes in platelet and red cell numbers.