Two-thirds of newly diagnosed bladder tumors are superficial transitional cell carcinomas. Local relapses occur in more than 2/3 of the patients after transurethral resection of the initial tumor, with a risk of muscle invasion in 1/3 of them. A retrospective study of 100 consecutive patients was performed to assess the role of intravesical BCG-therapy as a prophylaxis against the recurrence of superficial bladder cancer; the treatment protocol is described. Eighty-two patients had previously had recurrent tumors and 18 were given BCG directly because they had multiple, large, high grade or widespread tumors. Seventy patients (70 p. 100) tolerated the treatment well, but 6 experienced serious complications (3 systemic BCG infections and 3 bladder contractures). Of the 18 patients given BCG after their first tumor, 12 (66 p. 100) are tumor-free with a mean follow-up of 13 months. Of the 82 patients with previous recurrent tumors, 52 (63 p. 100) were improved by the treatment. Of the 100 patients, twelve tumors became muscle invasive and required more aggressive intervention. Intravesical BCG is effective in preventing recurrences of superficial bladder cancer. However, the side effects and the risk of muscle invasion require close monitoring.