The effectiveness of intravesical administration of bacillus Calmette-Guerin as a prophylaxis of superficial bladder cancer has been definitely demonstrated. On the other hand, therapeutic regimens, duration effects, efficacy of either maintenance cycles or repeated courses of therapy in case of failures are still controversial. We report the results achieved in 15 cases of carcinoma in situ of the bladder and in 48 cases of superficial bladder cancer (Ta-T1 stage of disease) with bacillus Calmette-Guerin immunotherapy. Our patients underwent an initial six week cycle and a following maintenance cycle with monthly administrations for one year. Median follow-up was 19 months (range 18-21 months). Patients with carcinoma in situ are now free of disease; on the contrary, patients with Ta-T1 tumors experienced 18 recurrences (28%). There was a marked decrease of recurrence rate when compared to previous local chemotherapy. We report in detail the adverse effects encountered and both histologic and ultrastructural findings observed after immunotherapy. Bacillus Calmette-Guerin therapy can influence positively the natural history of the disease but possible adverse effects should always be considered before starting the treatment.