PURPOSE: Neoadjuvant chemoradiotherapy is considered a standard approach for T3-4 M0 rectal cancer. In this situation, we compared neoadjuvant radiotherapy plus capecitabine with dose-intensified radiotherapy plus capecitabine and oxaliplatin. PATIENTS AND METHODS: We randomly assigned patients to receive 5 weeks of treatment with radiotherapy 45 Gy/25 fractions with concurrent capecitabine 800 mg/m(2) twice daily 5 days per week (Cap 45) or radiotherapy 50 Gy/25 fractions with capecitabine 800 mg/m(2) twice daily 5 days per week and oxaliplatin 50 mg/m(2) once weekly (Capox 50). The primary end point was complete sterilization of the operative specimen (ypCR). RESULTS: Five hundred ninety-eight patients were randomly assigned to receive Cap 45 (n = 299) or Capox 50 (n = 299). More preoperative grade 3 to 4 toxicity occurred in the Capox 50 group (25 v 1%; P < .001). Surgery was performed in 98% of patients in both groups. There were no differences between groups in the rate of conservative surgery (75%) or postoperative deaths at 60 days (0.3%). The ypCR rate was 13.9% with Cap 45 and 19.2% with Capox 50 (P = .09). When ypCR was combined with yp few residual cells, the rate was respectively 28.9% with Cap 45 and 39.4% with Capox 50 (P = .008). The rate of positive circumferential rectal margins (between 0 and 2 mm) was 19.3% with Cap 45 and 9.9% with Capox 50 (P = .02). CONCLUSION: The benefit of oxaliplatin was not demonstrated and this drug should not be used with concurrent irradiation. Cap 50 merits investigation for T3-4 rectal cancers.