Purpose To investigate the effect of adding concurrent chemotherapy (CT) to cetuximab plus radiotherapy (RT; CT-cetux-RT) compared with cetuximab plus RT (cetux-RT) in locally advanced squamous cell carcinoma of the head and neck (LA-SCCHN). Patients and Methods In this phase III randomized trial, patients with N0-2b, nonoperated, stage III or IV (nonmetastatic) LA-SCCHN were enrolled. Patients received once-daily RT up to 70 Gy with weekly cetuximab or with weekly cetuximab and concurrent carboplatin and fluorouracil (three cycles). To detect a hazard ratio (HR) of 0.64 for progression-free survival (PFS) with 85% power at a two-sided significance level of P = .05, 203 patients needed to be included in each arm. Results Four hundred six patients were randomly assigned to either CT-cetux-RT or cetux-RT. Patient and tumor characteristics were well balanced between arms, including p16 status. With a median follow-up of 4.4 years, the HR for PFS favored the CT-cetux-RT arm (HR, 0.73; 95% CI, 0.57 to 0.94; P = .015), with 3-year PFS rates of 52.3% and 40.5% and median PFS times of 37.9 and 22.4 months in the CT-cetux-RT and cetux-RT arms, respectively. The HR for locoregional control was 0.54 (95% CI, 0.38 to 0.76; P < .001) in favor of CT-cetux-RT. These benefits were observed regardless of p16 status for oropharynx carcinomas. Overall survival (HR, 0.80; P = .11) and distant metastases rates (HR, 1.19; P = .50) were not significantly different between the two arms. The CT-cetux-RT arm, compared with cetux-RT, had a higher incidence of grade 3 or 4 mucositis (73% v 61%, respectively; P = .014) and of hospitalizations for toxicity (42% v 22%, respectively; P < .001). Conclusion The addition of concurrent carboplatin and fluorouracil to cetux-RT improved PFS and locoregional control, with a nonsignificant gain in survival. To our knowledge, this is the first evidence of a clinical benefit for treatment intensification using cetux-RT as a backbone in LA-SCCHN.