Blood flow rates were measured using an isotopic dilution method in the main drainage segments of Brescia-Cimino arteriovenous fistulas in 9 pediatric patients who had undergone successful renal transplantation, 3 patients on hemodialysis and 2 patients with end-stage renal disease. Blood flow rates varied from 68 to 882 ml/min. The possibility that such high flow rates may have deleterious effects on limb growth, cosmetic appearance and cardiac function without any benefit to the patient is discussed.