Fluid kinetics were studied in children treated with continuous ambulatory peritoneal dialysis (CAPD) aged between 2 and 15 years. Dextran 70 was used as a volume marker. A 4-h dwell was studied with a dwell volume of 40 mg/kg. Transcapillary ultrafiltration was measured as well as marker clearance, which is the best available approximation of lymphatic absorption in the clinical setting. In 11 children in whom dialysate was used containing 1.36% glucose transcapillary ultrafiltration was 250 +/- 79 ml/4 h/1.73 m2 and marker clearance 236 +/- 101 ml/4 h/1.73 m2. In 13 children dialysed with 3.86% glucose, transcapillary ultrafiltration was 829 +/- 226 ml/4 h/1.73 m2 and marker clearance 307 +/- 176 ml/4 h/1.73 m2. These values are similar to those found in adult patients. There was a positive correlation between age and transcapillary ultrafiltration in the group receiving dialysate containing 3.86% glucose (r = 0.69, P = 0.009). There was no correlation between age and marker clearance. It is concluded that fluid kinetics in children and adults on CAPD are similar when corrected for body surface area. In young children transcapillary ultrafiltration is lower, probably because dwell volume is low in relation to peritoneal surface area in these children.