A randomised control trial of intravesical instillation of mitomycin C was conducted in 457 patients with cancer of the bladder that was confined to the submucosa on histological examination. The events studied were the recurrence free rate, the recurrence rate/year, and the number of new tumours developing/year. At the initial cystoscopy the tumours were completely resected and the patients randomised to have no instillation of mitomycin C, a single instillation of 40 mg in 40 ml of water at that cystoscopy, or a single instillation and then four further instillations. All patients had follow up cystoscopies every three months for the first year, twice in the second year, and yearly thereafter. After a median of 12 months, follow up information was available for 397 patients. Patients receiving both the single instillation of mitomycin C and the instillations at five cystoscopic examinations had significantly lower yearly recurrence rates and tumour rates than those in the control group, and the group receiving multiple instillations fared significantly better than those receiving a single instillation. The figures on progression to invasive cancer were too small to allow conclusions to be drawn.