Bacillus Calmette-Guerin (BCG) intravesical therapy represents a major advance in the treatment of superficial transitional cell carcinoma of the bladder. To date, however, the optimal treatment schedule must be defined and the toxicity related to the treatment is significant. The preliminary results of a randomized ongoing study performed to evaluate the effectiveness and relative toxicity of a low dose (75 mg.) BCG regimen in the treatment of superficial bladder cancer therapy are reported. A total of 126 patients (70 for prophylaxis of recurrent stages Ta and T1 papillary tumors and 56 for treatment of carcinoma in situ or with microinfiltration of the subepithelial connective tissue) underwent a 6-week course of 75 mg. BCG (Pasteur vaccine). An additional course was given in patients who failed to respond to the induction course. Maintenance therapy was administered in complete responders monthly for 1 year and then quarterly for 1 year. The prophylaxis group (transurethral resection plus BCG) was randomized versus transurethral resection alone (63 patients, control group). A complete response in the prophylaxis, control and therapy groups was observed in 74, 17 and 57% of the patients, respectively, while 4, 17 and 12.5%, respectively, experienced tumor progression. The additional course of therapy increased the response rate. On the contrary, previous unsuccessful intravesical chemotherapy did not affect the response rate. In regard to toxicity, irritative disturbances (27%) and fever (17%) appeared to be significantly decreased compared with the rates reported in the literature. No major complications were experienced. In conclusion, a low dose (75 mg.) Pasteur strain BCG regimen was effective as prophylaxis against recurrent superficial papillary tumors and as treatment of carcinoma in situ or with microinfiltration of the subepithelial connective tissue. Toxicity related to the treatment appeared to be low.