The aim of this study was to evaluate Kock reservoir function in children and adolescents operated for congenital urinary incontinence and to determine the complication rate. Reservoir function was investigated in 13 children (age range 10.8-16 years) and 7 adolescents (age range 16-18 years) through enterocystometry, enterocystoscopy and patient history. Patients were followed up for 3-10 years. The follow-up was reported as early (3 months-2 years) and late (2-10 years) postoperative periods. At early follow-up reservoir capacity was high in four patients and normal in the remaining patients. A low reservoir pressure was accompanied by high capacity and compliance. On enterocystometry first sensation for emptying was experienced in 82% of patients at early follow-up and 92% at late follow-up. Reservoir contractions were recorded in 60% of patients at early follow-up and 65% at late follow-up. The contractions were recorded at an average reservoir capacity of 270 ml at early follow-up and 340 ml at late follow-up. The complication rate was high in the child group compared with that in adolescents. Of 13 patients with at least one reservoir complication 10 were from the child group. Nipple dysfunction (angled nipple, prolapsed or stenosed stoma) occurred in 35% of patients, stones in 40% and bleeding during catheterization in 15%. Revision was performed in 38% of the child group and 15% of the adolescents. Reservoir perforation was observed in two patients at 6 and 9 years postoperatively. At late follow-up continence was excellent in 17 of 19 patients and good in 2. We conclude that the Kock reservoir is a good modality for urinary diversion, but the complication incidence is high in the child group (<16 years). Stability of the reservoir in terms of volume and low internal pressures was achieved one year after operation, except in the patients with infrequent reservoir emptying. A time-related increase in the reservoir sensitivity and contractility was reported on enterocystometry. Nipple dysfunction is common during the first two years after surgery, particularly in the child group. Stones may form 3-4 years after surgery. Satisfactory continence was achieved in all patients, although a revisional operation was necessary in some patients in order to obtain permanent continence.