26 patients with superficial bladder cancer underwent complete removal of tumors by transurethral resection and entered a prospective trial of immunoprophylaxis with Calmette-Guérin bacillus (BCG). The entire group had recurrent tumors that failed to respond to previous prophylaxis with thiotepa (13 patients) or other drugs (13 patients). Intravesical administration of BCG reduced the recurrence index and increased the disease-free interval. Side effects were mild and well tolerated. BCG seems to be effective in the prophylaxis of superficial bladder tumor recurrence after the failure of other drugs.