The authors prospectively assessed the independent association between weight loss and deterioration in quality of life (QOL) in patients treated by radiotherapy for head and neck cancer. In 533 head and neck cancer patients treated by curative radiotherapy, changes in weight and QOL were assessed between baseline (before radiotherapy) and follow-up (12 wk after the start of radiotherapy). Patients were categorized into 4 weight loss categories: 0%, 0.1%-5.0%, 5.1%-10.0%, and >10% weight loss. The association between weight loss and change in QOL was analyzed by linear regression analysis, adjusted for sociodemographic and tumor-related characteristics, and additionally for disease specific symptoms and tube feeding. Thirty percent of patients lost 0.1%-5.0% weight, 26% lost 5.1%-10.0% weight, and 24% lost >10% weight. Adjusted regression analyses showed a significant association between weight loss and deterioration of global QOL, physical functioning, social functioning, social eating, and social contact. After additional adjustment for disease-specific symptoms and tube feeding, weight loss (>10%) remained significantly associated with global QOL, social eating, and social contact (P < 0.05). More than 10% weight loss during and directly after radiotherapy has a significant impact on social eating, social contact, and QOL in head and neck cancer patients. Copyright © 2013, Taylor & Francis Group, LLC.