In 11 chronic haemodialysis patients we investigated whether the increase in haematocrit during recombinant human erythropoietin (rHuEPO) treatment might alter the long-term efficiency of haemodialysis. After correction of anaemia with rHuEPO (mean Ht 35 +/- 2% vs 19 +/- 2% at baseline) (p 0.001), mean predialysis creatinine and urea did not change, while predialysis phosphate (1.77 +/- 0.38 vs 1.51 +/- 0.29 mmol/l) were significantly increased (p 0.01). In six of the 11 rHuEPO treated patients a post- versus pre-dialysis haemoconcentration (haematocrit 44% vs 35%) not attributable to different ultrafiltration regimes, was observed. In these 6 patients mean predialysis phosphate, creatinine and urea tended to be higher, but not significantly, in comparison to he remaining 5 patients who did not haemoconcentrate. Dialyser clearances and total extractions for urea, creatinine, phosphate and inulin were compared to those of 11 matched haemodialysis patients with anaemia. No differences were observed either for small and middle molecule clearances or their extractions between rHuEPO and anaemic patients. In conclusion, dialysis efficiency is not affected if haematocrit values are kept about 35%.