Seventy-eight patients with superficial bladder cancer were entered into a randomized study. A streptococcal preparation, OK-432, was injected into bladder cancers before transurethral resection and instilled into the bladder for 6 months after resection to reduce the recurrence. The control group were only transurethrally resected, as usual. The recurrence rate for patients with primary disease was 3.6 in 100 months for the OK-432 group and 9.1 in 100 months for the control group (P less than 0.05). In the control group, multiple, large, sessile, and high-grade tumors had high recurrence rates. But in the OK-432 group, overall recurrence rates were low in both high-risk and low-risk patients. Supplementary immunotherapy is especially effective and is recommended in high-risk patients.