Objective: This study investigated the prevalence of the most common mental health symptoms in a large primary care patient population and characterized their determinants. Methods: Data came from a 2015-16 cross-sectional study of a primary care population in Switzerland. An investigator presented the study to patients in waiting rooms, and 1,103 completed a tablet-based questionnaire measuring stress in daily life, sleep disorders and anxiety and depressive symptoms. Diagnoses and treatments were recorded. Results: Moderate-to-high anxiety and depressive symptoms concerned 7.7% of patients; 27.6% felt stressed at least once a week; 17.2% had severe sleep disorders. Sociodemographic determinants were associated with psychiatric symptoms: female sex, young age, and frequency of consultations with a GP. Participants taking psychotropics had high levels of mental distress. Conclusion: Even though most patients were regularly monitored by their GP, a significant number of mental health problems were found. GPs should be provided with concrete tools to manage these patients better. Collaboration with mental health specialists should be encouraged in primary care settings.