After 6 months of (recombinant human erythropoietin) rHuEPO treatment we recently observed an increased peritoneal ultrafiltration (UF) (Nephron 53: 91, 1989). The aim of the present study was to investigate the long term effect of subcutaneous (SC) on dialysis efficiency in CAPD. Fourteen patients (11 female, 3 male, mean age 42.6, range 18-65 years) with renal anemia (HCT less than 28%) took part in the study. rHuEPO was administered s.c. twice weekly in an initial dose of 50 U/kg body weight. This dose was increased by 25 U/kg body weight every 4 weeks till the target HKT of 35% had been achieved. After 12 months of mean time of treatment (range 8-14 months) hematocrits had increased from 23.3 +/- 3.2 (x +/- SD) to 36.6 +/- 5.3% (p less than 0.005). UF improved from 0.70 +/- 0.22 to 1.03 +/- 0.47 ml/min (4 hr dwell time, 1.5% glucose monohydrate) (p less than 0.03). Increased UF resulted in an augmented creatinine clearance (p less than 0.05). No changes were observed in serum chemistries, body weight, pulse rate or cardiothoracic index. The observed increase in peritoneal ultrafiltration might be due to augmented mesenteric perfusion resulting from improved cardiac function. The sustained increase in UF after rHuEPO-induced correction of renal anemia may improve the fluid balance on CAPD patients.