We report the results of prospective randomized study which was designed to evaluate prophylactic effects of intravesical bacillus Calmette-Guerin (BCG) in the treatment of superficial transitional cell carcinoma of the bladder. A total of 44 cases who had no previous treatment of bladder cancer were randomly assigned to BCG or control groups after TUR. BCG group (23 cases) received intravesical instillation of 80 mg BCG (Tokyo strain) at one week intervals for 6 weeks, at two week intervals for 12 weeks, and at one month intervals for 20 months. In BCG groups, 3 cases had recurrence at 6 months and 1 case at 9 months, while the other 19 cases had no recurrent disease for 3 to 34 months (average 20.3 months) of follow-up. Control group (21 cases) had no further treatment after TUR. In control group, recurrence was seen at 3 months in 3 cases, at 6 months in 5 cases, at 9 months in 2 cases, at 12 months in 3 cases and at 21 months in 1 case. Only 7 cases in the control group were free of recurrence for 8 to 45 months (average 32.3 months) of follow-up. One and two year-recurrence rates in BCG group (18.4%, 18.4%) was significantly (p less than 0.01) lower than those in control group (63.2%, 68.9%). Among the complications of intravesical BCG were cystitis (76.2%), low grade fever (13.0%), and several days' persistent gross hematuria (13.0%). Most of these signs were self-limited and only in 2 cases instillation of BCG was discontinued.(ABSTRACT TRUNCATED AT 250 WORDS)