A dose‐response analysis was undertaken in a group of patients with clinically node‐negative carcinoma of the anal canal treated with a sphincter‐sparing approach. Twenty‐three patients underwent biopsy of the primary tumor, followed by concurrent radiation and chemotherapy. The chemotherapy regimen of 5‐fluorouracil and mitomycin C and radiation technique were uniform in all patients. The total radiation dose, given in daily fractions of 180–200 cGy, was escalated over the years as an institutional policy. The total radiation dose was ≤5,200 cGy (mean 4,550 cGy) in 11 cases treated before September 1989 and >5,200 cGy (mean 5,572 cGy) in 12 cases treated subsequently. The 3‐year actuarial local control rates were 53% vs. 100% for the low vs. high dose groups, respectively (P = 0.015). The 3‐year actuarial overall survival rates were 82% vs. 100% in the 2 groups, respectively. No increase in late toxicity in the high dose group has been observed compared to the low dose group. These results, albeit based on limited patient numbers, suggest that total radiation dose is an important determinant of long‐term tumor control in patients with localized anal canal carcinoma. A total radiation dose of approximately 5,500 cGy has led to excellent tumor control and sphincter function. © Wiley‐Liss, Inc. Copyright © 1994 Wiley‐Liss, Inc., A Wiley Company