Recombinant human erythropoietin does not increase clotting in vascular accesses. The incidence of vascular access clotting was evaluated over 5.25 years. The first 32 months served as a control period. During the second period of 31 months, recombinant human erythropoietin (epoetin) was used for an average duration of 13 months (range, 2-32 months) in 79 patients. The overall incidence of vascular access clotting decreased from a monthly rate of 0.06 to 0.03 events per patient-month over the 5 year period. Distribution of the number of events per patient did not differ between the two periods, with 55% to 60% of patients having no clotting episode. Patients with recurrent clotting (two or more events) accounted for 68% of episodes. During the second period, there were no differences in the incidence of vascular access clotting in epoetin treated patients vs untreated patients (0.38 events per patient-year vs. 0.46 events per patient-year, both slightly lower than in period 1 [0.52 events per patient-year]). It is concluded that epoetin does not increase vascular access clotting. 