STUDY OBJECTIVES: Patients seen in sleep clinics have been reported to have a high prevalence of mental disorders, but there is no recommended method for screening. We sought to assess the prevalence of mental disorders, using a validated screening instrument, among referrals to a sleep clinic and to determine whether referred patients with mental disorders were less likely to have an underlying diagnosis of sleep-disordered breathing. METHODS: Consecutive adult new referrals to a sleep clinic at a tertiary care center were invited to participate in this cross-sectional assessment of mental and sleep disorders. All patients were screened for mental disorders with the Primary Care Evaluation of Mental Disorders prior to their appointment with the sleep provider. Sleep disorders were diagnosed as per the sleep provider's judgment, usually by polysomnography. RESULTS: From 217 invited participants, 171 completed the survey and could be linked to clinical data. Eighty-one percent underwent polysomnography, and most (83%) had a diagnosis of sleep-disordered breathing. Thirty-eight patients (22%) had at least 1 mental disorder, and 17 (10%) had 2. Eleven percent of patients had major depression, 7% minor depression, 3% panic disorder, and 12% anxiety not otherwise specified. Patients with an underlying mental disorder were significantly less likely to have a diagnosis of sleep-disordered breathing, compared to those without an underlying mental disorder (66% vs 87%, P = .004). ABBREVIATIONS: MMPI, Minnesota Multiphasic Personality Inventory; MOS SF-6, Multiple Outcomes Study Short Form 6; OSA, Obstructive sleep apnea; PHQ-9, Patient Health Questionnaire-9; PHQ-15, Patient Health Questionnaire-15; PRIME-MD, Primary Care Evaluation of Mental Disorders; RDI, Respiratory Disturbance Index; SCL-90, Symptom Distress Check List. CONCLUSIONS: Mental disorders are common in adults referred to sleep clinics. Those with a mental disorder are less likely to have a diagnosis of sleep-disordered breathing. A negative sleep study should prompt clinicians to consider possible underlying psychiatric disease.