To evaluate the benefit of prophylactic inguinal irradiation (PII) in anal canal squamous cell carcinoma (ASCC). This retrospective study analyzed the outcome of 208 patients presenting with ASCC treated between 2000 and 2004 in four cancer centers of the south of France. The population study included 35 T1, 86 T2, 59 T3, 20 T4, and 8 T stage unknown patients. Twenty-seven patients presented with macroscopic inguinal node involvement. Of the 181 patients with uninvolved nodes at presentation, 75 received a PII to a total dose of 45-50 Gy (PII group) and 106 did not receive PII (no PII group). Compared with the no PII group, patients in the PII group were younger (60% vs. 41% of patients age <68 years, p = 0.01) and had larger tumor (T3-4 = 46% vs. 27% p = 0.01). The other characteristics were well balanced between the two groups. Median follow-up was 61 months. Fourteen patients in the no PII group vs. 1 patient in the PII group developed inguinal recurrence. The 5-year cumulative rate of inguinal recurrence (CRIR) was 2% and 16% in PII and no PII group respectively (p = 0.006). In the no PII group, the 5-year CRIR was 12% and 30% for T1-T2 and T3-T4 respectively (p = 0.02). Overall survival, disease-specific survival, and disease-free survival were similar between the two groups. In the PII group, no Grade >2 toxicity of the lower extremity was observed. PII with a dose of 45 Gy is safe and highly efficient to prevent inguinal recurrence and should be recommended for all T3-4 tumors. For early-stage tumors, PII should also be discussed, because the 5-year inguinal recurrence risk remains substantial when omitting PII (about 10%).