Animal studies have shown that metabolites of tryptophan can cause bladder cancer, and human observations reveal an appreciable incidence of abnormalities of tryptophan metabolism in patients with bladder cancer. It has been suggested that pyridoxine (vitamin B6) may correct this abnormality and prevent recurrences of superficial bladder cancers. Intravesical instillation of thiotepa has been used for more than fifteen years in the treatment of superficial bladder cancer, but no controlled trials have been done. We report here a prospective clinical trial of 121 patients with Stage I bladder cancer randomized to placebo, pyridoxine, or intravesical thiotepa. The percentages of patients with recurrences over the period of study were 60.4, 46.9, and 47.4 for the three groups, respectively, and did not differ significantly. However, if patients having recurrences during the first ten months or followed up less than ten months were excluded, pyridoxine was significantly better than placebo (P = 0.03). Thiotepa significantly reduced the recurrence rate compared with placebo (P = 0.016) or pyridoxine (P = 0.015). These results suggest that a new trial of pyridoxine should be undertaken in which the tryptophan metabolites are measured and that further study of intravesical instillation of chemotherapeutic agents is warranted.