A 67-year-old man was referred to an outpatient clinic of geriatric psychiatry because of persistent symptoms of depression and anxiety, accompanied by sleepiness. The latter had been evaluated multiple times in the general practice over several years; each time it was considered to be a symptom of depression. After referral, the patient was diagnosed with severe obstructive sleep apnoea (OSA), comorbid to a depressive and anxiety disorder. Retrospectively, we conclude that affective symptoms accompanying OSA and sleepiness were wrongfully interpreted as depression, but after having led to problems at work, they have triggered psychiatric comorbidity. Treatment of OSA in addition to the psychiatric disorders resulted in a full recovery over time. The delayed diagnosis of OSA has certainly diminished the patient's quality of life and might have precipitated the depressive disorder. Moreover, OSA poses patients at an increased risk of cardiovascular disease, hypertension, stroke and traffic accidents.