On the basis of 404 transurethral resections of tumours of the urinary bladder a report is given on complications and their prevention. Among 404 tumour resections 4 perforations of the bladder occurred (0.9%). The extraperitoneal perforations were simply paravesically drained; the intraperitoneal ones were oversewn and intraperitoneally drained. Post-operative bleeding was well controlled by means of a continuous wash via an additional thin catheter (Charr. 7-8) fitted into the urethra. Patients with existing infections of the urinary passages were traget-specifically treated with antibiotics. The careful coagulation of all bleeding vessels already when cutting is necessary. The good visibility thus achieved reduces the risk of perforation. If wash fluid begins to extravasate intravenous administration of 10% saline solution will prevent any hyponatraemia.