74 patients with duplex system operated on at the Zürich Children's Hospital between 1971 and 1986 are presented. The patients have been divided into four groups: 1. bifid system, 2. complete duplicated system, 3. duplicated system with ectopic ureter and 4. duplicated system with ureterocele. The most common complications were reflux and/or obstructive uropathy. Complications occurred in 39% of the patients in the first period (1971-1978) and in 20% in the second period (1979-1986). Patients with bifid systems and patients with duplicated systems with ectopic ureter showed good results at follow-up. Patients with duplicated systems and a ureterocele still show a high rate of complications. The decrease in complications during the second period is believed to be due to earlier and more precise preoperative diagnosis. The cross-trigonal reimplant showed better results in patients with duplicated systems than the Politano-Leadbetter procedure.