Peritoneal dialysis can result in significant protein losses through the dialysate effluent. Although protein loss in chronic ambulatory peritoneal dialysis has been examined, it has not been extensively studied in patients on continuous cycler peritoneal dialysis. Such losses can contribute to protein calorie malnutrition, especially in infants and children, many of whom are on continuous cycler peritoneal dialysis. We measured protein loss during continuous cycler peritoneal dialysis in patients ranging in age from 2 months to 18 years. There was an inverse correlation between body surface area and peritoneal protein loss, expressed both as milligrams of protein per kilogram body weight per day (P < 0.0001) and as milligrams of protein per meter square body surface area per day (P < 0.05). Peritoneal fluid protein losses in patients greater than 50 kg were similar to those previously reported in adults treated with chronic ambulatory peritoneal dialysis. In contrast, infants had nearly twofold greater peritoneal protein losses per meter square body surface area than older children weighing more than 50 kg. Such protein losses in infants impair normal growth and may contribute to permanent loss of growth potential. Infants on peritoneal dialysis require early and aggressive nutritional supplementation with higher caloric and protein intake to compensate for such dialysate protein losses and maximize growth.