BACKGROUND Erythropoietin (Epo) is an effective but expensive treatment for anaemia in patients with chronic renal failure. Hyporesponsiveness to Epo, particularly in haemodialysis patients, is most commonly due to a functional iron deficiency, which is difficult to monitor reliably. METHODS Forty-six stable haemodialysis patients, receiving Epo therapy, were commenced on regular low-dose intravenous iron (sodium ferric gluconate complex) at a dose of 62.5 mg/5 ml given as a slow injection post-dialysis twice weekly, weekly, or fortnightly, according to their serum ferritin levels. Haemoglobin, serum ferritin, Epo dose, and iron dose were measured at 6-weekly intervals over a 6-month period. RESULTS At the beginning of the study, 12 patients in the group had ferritin levels of less than 100 microg/l, and were thus considered to potentially have an absolute iron deficiency. The study group was therefore split into two subgroups for the purpose of analysis, i.e. the 12 patients with ferritin levels of less than 100 microg/l at the start of the study or 'low ferritin group', and the remaining 34 patients with ferritin levels of greater than 100 microg/l at the start of the study or 'normal ferritin group'. In the low ferritin group (n=12), intravenous iron therapy increased serum ferritin levels, and produced a significant rise in haemoglobin, and a significant reduction in Epo dose. (Ferritin pre-iron, median (range) 68 (20-96) microg/l; post-iron, 210.5 (91-447) microg/l, P<0. 003, Wilcoxon. Haemoglobin pre-iron, 10.05 (8.2-11.9) g/dl; post-iron, 11.0 (9.9-11.9) g/dl, P<0.03. Epo dose pre-iron, 9000 (4000-30 000)-i.u./week, P<0.05). Similar results were obtained in the normal ferritin group (n=34) following intravenous iron therapy, with significant increases in serum ferritin levels and haemoglobin concentrations, and a significant reduction in Epo dose. (Ferritin pre-iron, 176 (103-519) microg/l; post-iron, 304.5 (121-792) microg/l, P<0.0001. Haemoglobin pre-iron, 9.85 (6.5-12.8) g/dl; post-iron: 11.25 (9.9-13.3) g/dl, P<0.0001. Epo dose pre-iron, 6000 (2000-15 000) i.u./week; post-iron, 4000 (0-15 000)-i.u./week, P<0. 005). CONCLUSIONS Regular intravenous iron supplementation in haemodialysis patients improves the response to Epo therapy.