PURPOSE: The aim of this study was to define the scales and test the validity, reliability, and sensitivity of the European Organization for Research and Treatment of Cancer Quality of Life Questionnaire (EORTC QLQ)-H&N35, a questionnaire designed to assess the quality of life of head and neck (H&N) cancer patients in conjunction with the general cancer-specific EORTC QLQ-C30. PATIENTS AND METHODS: Questionnaires were given to 500 H&N cancer patients from Norway, Sweden, and the Netherlands as part of two prospective studies. The patients completed the questionnaires before, during (Norway and Sweden only), and after treatment, yielding a total of 2070 completed questionnaires. RESULTS: The compliance rate was high, and the questionnaires were well accepted by the patients. Seven scales were constructed (pain, swallowing, senses, speech, social eating, social contact, sexuality). Scales and single items were sensitive to differences between patient subgroups with relation to site, stage, or performance status. Most scales and single items were sensitive to changes, with differences of various magnitudes according to the site in question. The internal consistency, as assessed by Cronbach's alpha coefficient, varied according to assessment point and within subsamples of patients. A low overall alpha value was found for the speech and the senses scales, but values were higher in assessments of patients with laryngeal cancer and in patients with nose, sinus, and salivary gland tumors. Scales and single items in the QLQ-H&N35 seem to be more sensitive to differences between groups and changes over time than do the scales and single items in the core questionnaire. CONCLUSION: The QLQ-H&N35, in conjunction with the QLQ-C30, provides a valuable tool for the assessment of health-related quality of life in clinical studies of H&N cancer patients before, during, and after treatment with radiotherapy, surgery, or chemotherapy.