A marked proportion of primary care patients have mental disorders and problems that remain unrecognized by the patients and their general practitioners. There is furthermore a great variation in the physicians' ability to detect mental disorders. The aim of the present study was to find out the overall prevalence of mental disorders among patients receiving primary health care. The material consisted of 1000 randomly selected adult patients attending primary care facilities in Turku. The mental disorders were assessed by using the Symptom Checklist (SCL-25) and by general practitioners. According to the SCL-25, one fourth of the sample had a mental disorder. Only two fifths of them could be identified by the general practitioners.