As a component of the open-label, multicenter National Cooperative Recombinant Human Erythropoietin (Epo) Study, the health-related quality-of-life effects of Epo therapy were assessed in 484 dialysis patients who had not previously been treated with Epo therapy (New-to-Epo) and 520 dialysis patients who were already receiving Epo therapy at the time of study enrollment (Old-to-Epo). Using scales from the Medical Outcomes Study 36-item Short Form Health Survey (SF-36), health-related quality of life was assessed on study enrollment (baseline) and at an average of 99 days follow-up. At baseline, SF-36 scores for Old- and New-to-Epo patients were well below those observed in the general population, reflecting substantial impairments in functional status and well-being among patients with chronic renal failure. Significant improvements from baseline to follow-up were observed among New-to-Epo patients in vitality, physical functioning, social functioning, mental health, looking after the home, social life, hobbies, and satisfaction with sexual activity (P < 0.05 for each). The mean improvements in hematocrit values among New-to-Epo and Old-to Epo patients were 4.6 and 0.3, respectively. At the time of follow-up, SF-36 scores for New-to-Epo patients were comparable with those observed among Old-to-Epo patients, whose scores did not change significantly from baseline to follow-up. Analysis of the relationship between Epo therapy, hematocrit values, and health-related quality of life suggest that some of the beneficial quality-of-life effects of Epo are mediated through a change in hematocrit level.