Background and Objectives: Sleep disturbance is prominent in many neuropsychiatric disorders and may precipitate or exacerbate a range of psychiatric conditions. Few studies have investigated sleep disordered breathing and in particular obstructive sleep apnoea in community psychiatric patients and the commonly used screening instruments have not been evaluated in patients with psychiatric disorders. The objective is to evaluate the prevalence of sleep disordered breathing in a community cohort with chronic mental illness on long term psychotropic medication, and to assess the effectiveness of commonly used screening instruments to detect abnormal sleep. Methods: 52 patients completed sleep questionnaires and 50 undertook overnight oximetry. Results: 52% (n = 26) had sleep-disordered breathing; 20% (n = 10) had moderate/severe sleep apnoea. The Epworth Sleepiness Score and the Pittsburgh Sleep Quality Inventory did not predict sleep disordered breathing. Conclusions: Patients with psychiatric disorders in the community have a high rate of undiagnosed sleep disordered breathing, which is not reliably detected by established sleep disorder screening questionnaires.