In a randomized trial 2 groups of 28 patients who had undergone transurethral resection of bladder tumors were treated with 20 mg. mitomycin C alone or with 200,000 units hyaluronidase to determine whether adjuvant hyaluronidase would improve tumor recurrence rates. Patient groups were comparable statistically. In the group receiving additive hyaluronidase the percentage of tumor recurrences was decreased significantly (p less than 0.05). Side effects were not increased. Thus, adjuvant hyaluronidase appears to have a role in the metaphylaxis of bladder tumors. The potential reduction of the hyaluronidase dose without loss of protective action and the role of additive hyaluronidase in the systemic treatment of metastatic urothelial tumors remain to be investigated.