A controlled prospective study in 100 patients evaluated the efficacy of intravesical bacillus Calmette-Guérin (BCG) administration as prophylactic treatment on tumor recurrence and tumor progression rate after endoscopic resection of superficial transitional cell carcinoma of the bladder. There were 27 recurrences in 22 of 67 evaluable patients (33%) who received BCG, compared to 27 recurrences in 19 of 33 control patients (58%) (p < 0.05). The mean follow-up periods for the tumor-free patients in the BCG and control groups were 29 and 30 months, respectively, while the mean times to tumor recurrences for the above groups were 13.36 ± 6 and 9.94 ± 5 months, respectively (p < 0.05). The recurrence rates per 100 patient months for the BCG and control patients were 1.69 and 4.41 recurrences, respectively (p < 0.05), while 7 patients of the BCG group showed recurrent tumors of higher stage or grade, compared to 13 of the controls (p < 0.05). This study confirms that Pasteur strain BCG is safe and efficacious in the prevention of superficial bladder tumor recurrence and tumor progression.