Epidemiological and laboratory studies suggest that vitamin supplements may be helpful in the prevention of some cancers but clinical trials to date have failed to demonstrate protection with naturally occurring vitamins. Without substantiation of the highly touted benefits of vitamins, few physicians who care for cancer patients have recommended their use. A total of 65 patients with biopsy confirmed transitional cell carcinoma of the bladder enrolled in a randomized comparison of intravesical bacillus Calmette-Guerin (BCG) with or without percutaneous administration was also randomized by closed envelope to therapy with multiple vitamins in the recommended daily allowance (RDA) versus RDA multivitamins plus 40,000 units vitamin A, 100 mg. vitamin B6, 2,000 mg. vitamin C, 400 units vitamin E and 90 mg. zinc. The addition of percutaneous BCG did not significantly lessen tumor recurrence but recurrence after 10 months was markedly reduced in patients receiving megadose vitamins. The 5-year estimates of tumor recurrence are 91% in the RDA arm and 41% in the megadose arm (p = 0.0014, Mantel-Cox). Overall recurrence was 24 of 30 patients (80%) in the RDA arm and 14 of 35 (40%) in the high dose arm (p = 0.0011, 2-tailed Fisher's exact test). Megadose vitamins A, B6, C and E plus zinc decrease bladder tumor recurrence in patients receiving BCG immunotherapy. Further research will be required to identify which ingredient(s) provide this protection.