In a Danish community psychiatric service the patients referred from general practitioners (GPs) to the community psychiatric service (CPS) are compared with matched individuals with conspicuous psychiatric morbidity treated in general practice only. The psychopathology of the referred patients is more severe, as estimated by two different methods. They make fewer social contacts and their work situations are unfavourable. It was found that to a great extent the GPs refer their patients with mental disorders to the CPS so that the 'filter' between the GP and the CPS is very permeable. Of the patients treated by the GPs only (the matched group), no more than 54% were assessed by a psychiatrist as psychiatric 'cases'. Psychopathology thus only partly determines the GPs' assessment of psychiatric 'caseness', in which social impairment plays an important part. The GPs diagnose more mental disorder than the psychiatrists, possibly because of an intimate acquaintance with the anamnesis.