The prevalence of obstructive sleep apnoea (OSA) is estimated to be 3–7.5% in men and 2–3% in women. In mentally ill population it is even higher, as these patients are a high risk OSA group. The aim of the paper was a review of literature about the prevalence of sleep apnoea in patients with schizophrenia, bipolar disorder and recurrent depressive disorder.The available data show that OSA is present in 15–48% of patients with schizophrenia, 21–43% of patients with bipolar disorder and 11–18% of patients with recurrent depressive disorder. The lack of diagnosis of OSA in people with mental illnesses has multiple negative consequences. The symptoms of sleep apnoea might imitate the symptoms of mental illnesses such as negative symptoms of schizophrenia and symptoms of depression, they might as well aggravate the cognitive impairment. A number of the drugs used in mental disorders may aggravate the symptoms of OSA. OSA is as well the risk factor for cardiovascular and metabolic diseases which are a serious clinical problem in mentally ill people and contribute to shortening of their expected lifespan. From the point of view of the physicians treating OSA it is important to pay attention to the fact that co-existing depression is the most common reason for resistant daytime sleepiness in OSA patients treated effectively with Continuous Positive Airway Pressure (CPAP). CPAP therapy leads to significant improvement of mood. However, in schizophrenia and bipolar patients it may rarely lead to acute worsening of mental state, exacerbation of psychotic symptoms or phase shift from depression to mania.