Several factors influence the efficacy of the action of human recombinant erythropoietin during treatment of anaemia in haemodialysis patients. We carried out a six-month prospective study of 23 stable patients who had been on haemodialysis for at least one year to attempt to evaluate those factors modifying the dose of the hormone to attain a similar haematocrit, such as use or not of angiotensin converting enzyme inhibitors, hepatitis C virus positive or negative, age older or younger than 60 years, acquired cystic kidney disease or not, and sex. The patients were treated with subcutaneaous erythropoietin for over a year to attain a haematocrit of 35%, intravenous iron to reach plasma ferritin levels > 250 ng/ml and a transferrin saturation index > 20%, folic acid and group B vitamins. Parameters studied included age, time and duration of haemodialysis, Kt/V, albumin, haematocrit, erythropoietin in U/kg/week, intact PTH, hepatitis C virus, PCR of the hepatitis C virus, transaminases, ferritin, transferrin saturation index, folic acid, vitamin B12, and aluminium. No statistically significant differences were seen between the patients with and without hepatitis or in age or acquired cystic kidney disease and sex in the hormone dose given to achieve similar levels of haematocrit. Higher doses of erythropoietin were necessary in those patients treated with antihypertensive agents (71 +/- 25 vs 44 +/- 25 U/kg/week; p < 0.05). There were no differences between groups in factors known to cause resistance to the action of the hormone. The most important conclusions from this study concern the cost-benefit relation of treating hypertensive patients on haemodialysis with angiotensin converting enzyme inhibitors and erythropoietin.