Background: Helical intensity‐modulated radiotherapy (H‐IMRT) provides excellent limitation of dose to tissues not requiring treatment, although acute toxicity still occurs. The present study aimed to determine how treatment‐related acute toxicities affect nutrition outcomes in patients with head and neck cancer. Methods: A prospective observational study was conducted in 194 patients undergoing curative intent H‐IMRT with or without other treatment modalities. Weight outcomes (kg) and acute toxicity and dysphagia data were collected during treatment using Common Toxicity Criteria for Adverse Effects (CTCAE), version 4.0. Results: Significant weight loss (> 10%) was observed in 30% of high nutritional risk patients and 7% of low nutritional risk patients. Nausea, adjusted for baseline dysphagia, in high nutritional risk patients and nausea, dysphagia and pharyngeal mucositis in low nutritional risk patients were significant factors in explaining the percentage loss in baseline weight to treatment completion. Conclusions: Significant weight loss remains an issue during treatment, despite improvements in radiotherapy technology and high‐level multidisciplinary care. Key points: Acute toxicities continue to have significant implications on nutrition outcomes for patients with head and neck cancer (HNC) during treatment.Nausea is a strong predictor for weight loss during treatment, with worse severity of nausea implicated in a greater weight loss over time.Clinically significant weight loss during treatment remains a persistent outcome for patients with HNC receiving definitive or adjuvant helical intensity‐modulated radiotherapy. [ABSTRACT FROM AUTHOR]