BACKGROUND: Although psychiatric disorders are highly prevalent in the community, many patients with a psychiatric morbidity remain unidentified as such in primary care. OBJECTIVE: The aim of this study was to analyze which clinical and sociodemographic characteristics of patients with psychiatric morbidity are related to general practitioners' (GPs) diagnosis of mental illness. METHODS: A 1-year naturalistic survey of primary care contacts of patients with a Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition (DSM-IV) diagnosis of affective disorder, anxiety disorder or alcohol abuse was carried out. RESULTS: Of the patients with a DSM-IV diagnosis, 10% did not visit their GP at all during 1 year, 40% visited their GP but were only diagnosed as having somatic diagnoses and 50% were given a psychological or social diagnosis at least once during 1 year. Affective disorders are more frequently diagnosed than anxiety disorders or alcohol abuse. The chances of psychological GP diagnosis increase with the number of GP contacts. CONCLUSIONS: GPs appear to have few indications to help them distinguish patients with a psychiatric morbidity from others, as long as the patients themselves do not express more explicit clues to their condition. Public mental health education and a better public relations policy are badly needed.