Objectives: To evaluate the prevalence of obstructive sleep apnoea syndrome (OSAS) in a population of patients treated for an advanced oropharyngeal cancer (AJCC Stage III or IV), depending on treatment strategy, and to evaluate its impact on quality of life. Design: Prospective cohort study. Setting: University Teaching Hospital of La Conception, Marseille, France. Participants: Fifty-one disease-free patients were included. Forty-one patients received a combined chemoradiotherapy, while 10 patients were treated by surgery followed by chemoradiotherapy. Main outcome measures: Every patient underwent a formal sleep consultation and was asked to complete the Epworth Sleepiness Scale and EORTC QLQ C-30 and the EORTC H&N 35 questionnaires. A home overnight respiratory polygraphy was performed in every subject. Results: The mean time between the end of cancer treatment and the OSAS analysis was 54.04 months [20; 84]. An OSAS was found in 25.49% of our patients. There was no significant difference between patients treated with either surgery (30%) or CRT (24.39%), P=.79. The EORTC QLQ C-30 questionnaire showed a significant difference between positive and negative OSAS groups in the Global Health Status Scale (50.64 vs 67.11, P=.02) and in the fatigue item (35.04 vs 17.25, P=.03). Conclusions: Our population with advanced oropharyngeal cancer, whatever the treatment strategy it may be, was at risk of developing OSAS with negative impact on quality of life. A routine screening and treatment of OSAS seems necessary to improve the quality of life of patients treated for advanced oropharyngeal cancer.