The aim was to assess the significance of dry mouth upon awakening as a symptom of obstructive sleep apnea (OSA). The participants were 668 consecutive adults referred for polysomnographic evaluation (PSG) because of snoring and suspected OSA, and 582 adults who were attending a general health check-up. Data were obtained from self-administered questionnaires and PSG evaluation. The participants were asked to answer the following question: 'During the last month, did you experience waking up in the morning with a dry mouth?'. The response scale consisted of five categories: 'never', 'rarely', 'sometimes', often', or 'almost always'. We classified patients as having dry mouth upon awakening complaint only if they reported experiencing the symptom 'almost always'. The prevalence of dry mouth upon awakening was twofold higher in patients with OSA (31.4%) than in primary snorers (16.4%, P < 0.001), and increased linearly from 22.4%, to 34.5%, and 40.7% in mild, moderate, and severe OSA respectively (P < 0.001). The prevalence of dry mouth upon awakening in the control group was 3.2%. Logistic regression results indicated that this symptom significantly differentiated OSA patients from primary snorers after adjusting for age, BMI, gender, hypertension, and other classical OSA symptoms (OR 2.33, 95% CI 1.34-4.07). Dry mouth upon awakening appears as a significant symptom of OSA. We suggest that increased sleep time spent with an open mouth is a likely explanation for these findings.