A double-blind trial was conducted in 138 patients with superficial transitional cell carcinoma of the bladder following transurethral resection to evaluate the prophylaxis of recurrence by an oral Lactobacillus casei preparation (BLP). Patients were stratified into the following 3 subgroups: (A) with primary multiple tumors; (B) with recurrent single tumors, and (C) with recurrent multiple tumors. In each group, patients were randomly allocated to receive BLP or placebo. BLP showed a better prophylactic effect in subgroups A and B than placebo, whereas no significant difference was observed in subgroup C. Cox multivariate analysis showed that the outcome with BLP was significantly better than with placebo (p = 0.01). Slight and tolerable adverse reactions occurred in 3 patients receiving BLP and in 3 of the placebo-treated patients. Oral administration of BLP was thus safe and effective for preventing recurrence of superficial bladder cancer.