Purpose Squamous cell carcinoma of the anal canal (SCCAC) is characterized by high locoregional failure (LRF) rates after definitive chemoradiation (CRT), associated with anogenital human papilloma virus, and often appears in HIV infection. Because cetuximab enhances the effect of radiation therapy in human papilloma virus-associated oropharyngeal SCC, we hypothesized that adding cetuximab to CRT would reduce LRF in SCCAC. Methods Forty-five patients with stage I to III SCCAC and HIV infection received CRT: 45 to 54 Gy radiation therapy to the primary tumor and regional lymph nodes plus eight once-weekly doses of concurrent cetuximab and two cycles of cisplatin and fluorouracil. The study was designed to detect at least a 50% reduction in 3-year LRF rate (one-sided a, 0.10; power, 90%), assuming a 35% LRF rate from historical data. Results The 3-year LRF rate was 42% (95% CI, 28% to 56%; one-sided P = .9) by binomial proportional estimate using the prespecified end point (LRF or alive without LRF and followed , 3 years), and 20% (95% CI, 10% to 37%) by Kaplan-Meier estimate in post hoc analysis using definitions and methods consistent with historical data. Three-year rates by Kaplan-Meier estimate were 72% (95% CI, 56% to 84%) for progression-free survival and 79% (95% CI, 63% to 89%) for overall survival. Grade 4 toxicity occurred in 26%, and 4% had treatment-associated deaths. Conclusion HIV-associated SCCAC is potentially curable with definitive CRT. Although addition of cetuximab may result in less LRF, the 20% recurrence and 26% grade 4 toxicity rates indicate the continued need for more-effective and less-toxic therapies.