The usefulness of the peritoneal equilibration test (PET) in children is unknown. The relationship between transcapillary ultrafiltration and PET was investigated in order to evaluate the usefulness of PET in children. PET was performed on 14 patients undergoing peritoneal dialysis. Their age and bodyweight ranged from 3.8 to 23.6 years and 10.2 to 55.8 kg, respectively. The patients were divided into two groups according to bodyweight; group A patients weighed < or = 40 kg (n = 7) and group B patients weighed > 40 kg (n = 7). There was no significant difference in the mean infusion volume per bodyweight between the two groups, but the mean infusion volume per body surface area was smaller in group A than in group B. Group A showed a more rapid equilibration of dialysate glucose and creatinine than group B. Higher normalized mass transfer area coefficients were evident in group A. In spite of the lower effective glucose gradient in group A, the transcapillary ultrafiltration capacity (TUFC) showed no difference between the two groups. When the slope indices of the regression equations between the two groups were compared, the slopes of the regression in the relationship between TUFC and dialysate (D) ratios D/Do glucose or D/P creatinine in group A were steeper than those in group B. Results of the present study indicate that the larger peritoneal area to infusion volume in patients with smaller body size results in both a rapid equilibration of solutes and sufficient transcapillary ultrafiltration.