A randomized controlled study was conducted in patients with superficial bladder cancer in order to investigate the safety of an orally administered (3 g/day) Lactobacillus preparation, biolactis powder (BLP), and its preventive effect on the recurrence after transurethral resection of the bladder tumor (TUR-Bt). Of 58 cases enrolled in the study 23 cases in the BLP group and 25 cases in the control group completed the study. There were no significant differences in the patient characteristics for the complete cases between the two groups. Comparison of the disease-free duration by the Kaplan-Meier method revealed that the 50% recurrence-free interval after TUR-Bt was prolonged by BLP treatment (350 days) to 1.8 times that in the control group (195 days). A significant difference between the groups was detected by the long-rank test (p = 0.03). No adverse side effect was observed. The results suggest that oral administration of BLP is useful for the prevention of the recurrence of superficial bladder cancer.