The National Bladder Cancer Group undertook a study to determine the effectiveness and toxicity of mitomycin in patients who failed on thiotepa. A total of 117 patients with residual superficial transitional cell bladder cancer (Ta, T1, Tis) who had previously failed on intravesical thiotepa were treated with 40 mg of mitomycin instilled intravesically weekly for eight weeks. Four to six weeks after the last treatment, tumor response was evaluated by cystoscopy, biopsy of the site of the index lesion, and cytology. In 57 patients (48.7%), visible tumor had been ablated. Results of cystoscopy, biopsy, and cytology were negative in 32 (27.4%) patients. Eleven patients (9.4%) had no visible tumor and negative cytology unconfirmed by biopsy. In 14 patients (12%) who had complete destruction of the tumor at cystoscopy, and biopsy specimen was negative for tumor, cytology was positive, indicating a partial response. Six patients (5.1%) withdrew from the study before the first evaluation because of local toxicity (cystitis).