Purpose: Definitive chemoradiation (CR) has replaced radical surgery as the preferred treatment of epidermoid carcinoma of the anal canal. To determine the importance of mitomycin (MMC) in the standard CR regimen and to assess the role of salvage CR in patients who have residual tumor following CR, a phase III randomized trial was undertaken by the Radiation Therapy Oncology Group (RTOG)/Eastern Cooperative Oncology Group (ECOG). Patients and Methods: Between August 1988 and December 1991, 310 patients were randomized to receive either radiotherapy (RT) and fluorouracil (5-FU) or radiotherapy, 5-FU, and MMC. Of 291 assessable patients, 145 received 45 to 50.4 Gy of pelvic RT plus 5-FU at 1,000 mg/m2/d for 4 days, and 146 received RT, 5-FU, and MMC (10 mg/m2 per dose for two doses). Patients with residual tumor on posttreatment biopsy were treated with a salvage regimen that consisted of additional pelvic RT (9 Gy), 5-FU, and cisplatin (100 mg/m2). Results: Posttreatment biopsies were positive in 15% of patients in the 5-FU arm versus 7.7% in the MMC arm (P = .135). At 4 years, colostomy rates were lower (9% v 22%; P = .002), colostomy-free survival higher (71% v 59%; P = .014), and disease-free survival higher (73% v 51%; P = .0003) in the MMC arm. A significant difference in overall survival has not been observed at 4 years. Toxicity was greater in the MMC arm (23% v 7% grade 4 and 5 toxicity; P ≤ .001). Of 24 assessable patients who underwent salvage CR, 12 (50%) were rendered disease-free. Conclusion: Despite greater toxicity, the use of MMC in a definitive CR regimen for anal cancer is justified, particularly in patients with large primary tumors. Salvage CR should be attempted in patients with residual disease following definitive CR before resorting to radical surgery.