The response to erythropoietin (Epo) is dose dependent but, for various poorly understood reasons, variable. In a cross-sectional study we determined the Epo requirement of 60 patients in a dialysis population to identify those patients requiring a high dose of Epo, and ascertained the reasons for higher requirements, paying particular attention to the effect of previous transplantation. All 289 patients attending a single centre were surveyed. Of these, 164 were receiving renal replacement therapy by continuous ambulatory peritoneal dialysis (CAPD) and 125 were on haemodialysis (HD). Patients on HD needed more Epo than those on CAPD (129.0 +/- 14.9 U/kg/week versus 86.9 +/- 10.7 U/kg/week, P < 0.05). However, this difference was accounted for by a subgroup of patients who had a previously failed transplant. The Epo requirement in those patients on HD with a failed transplant was significantly greater than those on HD who had never been transplanted (164.0 +/- 24.5 U/kg/week versus 96.6 +/- 11.9 U/kg/week, P < 0.05). The seven patients who retain their transplanted kidney had the highest Epo requirement of all (213.4 +/- 46.6 U/kg/week). These studies have shown that previous transplantation is a significant determinant of Epo requirement upon return to dialysis. They also show that it is necessary to 'correct' for the effect of previous transplantation when investigating generally accepted determinants of Epo need. Interpretation of previously published studies needs to take account of this.