The study covers a series of 221 patients with pTa or pT1 vesical tumours who were treated by RTU and had an endovesical cytostatic agent instilled prophylactically for a year. Before beginning the tumour resection, biopsies in map with cold forceps were routinely taken from the trigone, side walls, bottom walls and vesical cup. It was decided to monitor the patients by performing cystoscopies and biopsies in map with cold forceps every quarter. During the second and third year the endoscopic and bioptic monitoring was done every six months. During the time when the cytostatic agents were instilled, a significant decrease (p less than 0.01) in the percentage of patients with 'CIS' was observed. After discontinuing the instillation, the percentage of patients with 'CIS' reversed to similar figures as seen prior to the study. It can be deduced from this observation that prophylaxis can be effective against those elements of the urothelial disease which influence the arrival of new events and the increase of the tumoral stage, and that the control of those factors disappears when the prophylaxis is discontinued. The study illustrates that prophylaxis in the urothelial disease should be maintained indefinitely.