Superficial bladder tumors (stage Ta, T1, and Tis) may progress to invade the bladder muscle and cause death from metastatic cancer. Transurethral tumor resection (TURB) is the standard therapy for such tumors, but surgery alone may not prevent tumor progression. Intravesical therapy is widely used as an adjunct to TURB. Bacillus Calmette-Guérin (BCG) is the most active intravesical agent, but whether BCG prevents tumor progression and death from bladder cancer is unknown. Between 1978 and 1981, 86 high-risk patients with superficial bladder cancer were randomly assigned to receive either TURB (n = 43) or TURB plus BCG (n = 43). Adverse tumor features for progression were equally distributed between the two groups. BCG was administered weekly for 6 weeks. Patients were evaluated every 3 to 6 months thereafter for progression to muscle invasion or metastasis. Control (TURB) patients with recurrent superficial tumors were eligible for crossover to the BCG arm. All patients have been monitored until event or for a minimum of 10 years (range, 10 to 14). The 10-year progression-free rate was 61.9% (95% confidence interval CI, 47.2% to 76.7%) for patients treated with BCG and 37% (95% CI, 22.9% to 53.1%) for control patients. The median progression-free interval was not reached for the BCG group and was 46 months for the control group (P = .0063). Of 18 control patients crossed over to BCG (median, 29 months), 15 did not show tumor progression. TURB plus BCG resulted in a 10-year disease-specific survival rate of 75%, compared with 55% with TURB alone (P = .03). This study shows that intravesical therapy with BCG delays tumor progression and death from tumor in patients who present with superficial bladder cancer.