A total of 78 patients with superficial bladder carcinoma were prospectively randomized to two groups following complete transurethral resection (TUR). Each received 12 intravesical instillations of 10(7) units interferon A or 120 mg BCG Connaught for 1 year starting 6 weeks post-TUR. After a mean observation period of 24 (13-31) months in the BCG and 25 (6-32) months in the IFN group 5/32 (15.6%) recurrences in the BCG versus 21/35 (60%) in the IFN group were observed (P = 0.0003). In the IFN group 18.4% of the patients had dysuria and 2.6% fever; in the BCG group 35% had fever, 60% cystitis, 1 patient granulomatous epididimoorchitis and 1 patient pneumonitis with granulomatous prostatitis. With our instillation regimen interferon A had few side effects but also no prophylactic effect, whereas BCG had tolerable-seldom severe--side effects and was very effective in preventing recurrences. Perhaps IFN should be given earlier after TUR and in a higher dosage.