Key PointsQuestionWhat is the association of overweight and obese body mass index (BMI) with posttreatment response, tumor recurrence, and survival outcomes among patients with head and neck cancer who underwent chemoradiotherapy?FindingsIn this cohort study involving 445 patients, both overweight and obese BMI were associated with complete metabolic response after chemoradiotherapy. Only overweight BMI was associated with improved overall survival, progression-free survival, and reduction in locoregional failure.MeaningThis study suggests that overweight BMI is an independent factor favorably associated with complete metabolic response after chemoradiotherapy, survival, and locoregional failure.