The Mini Finland Health Survey was an extensive epidemiological study of the Finnish population aged 30 or over; the prevalence of mental disorders was one aspect studied. Prevalence of symptoms in the General Health Questionnaire as well as the prevalence of self-perceived and clinically assessed mental disorders was studied. The total prevalence of clinically assessed mental disorders was 17.4%, 14.8% in the men and 19.5% in the women. A definite disorder was observed for 11.7% of the subjects. About half of the subjects suffering from a mental disorder according to clinical assessment also reported a self-perceived disorder. Of various diagnoses, the most common were phobic and anxiety neurosis (6.2%) and neurotic depression (4.6%). The prevalence of schizophrenia was 1.3%. The highest prevalence was found in the ages 50 to 64 years. The prevalence of psychoses was highest in northern and eastern Finland, and that of neuroses in southern Finland.