The purpose of this study was to estimate the prevalence of mental disorders in a random sample of three age groups born in 1931, 1951 and 1971 and living in the Greater- Reykjavik Area. From the original sample of 300 in each birth cohort 805 were contacted and asked to take part in a survey of mental health. Of those 52% participated. The instrument used for diagnosing mental disorders was the CIDI-Auto. The lifetime prevalence of any ICD-10 disorder was found to be 49.8%. The most frequent diagnoses were Mental and behavioral disorders due to use of tobacco (23,6%), Somatoform disorders (19.0%) and Mental and behavioral disorders due to use of acohol (10.8%).Any anxiety disorder was found in 14.4% and any mood disorder in 13.0%. The one-year prevalence for any disorder was 19.7%, for tobacco use disorder 6.5%, alcohol use disorder 6%, somatoform disorders 10.3%, anxiety disorders 5.5% and mood disorders 2.6%. Alcohol use disorders are more prevalent among men but somatoform disorders, anxiety disorders and mood disorders among women. The prevalence rates of mental disorders found in this study is the same or lower than rates found in other comparable studies. There is no indication of an increase in the rate of mental disorder in Iceland. Keywords: mental disorder, anxiety, depression, epidemiology, CIDI. Correspondence: JÃ³n G. StefÃ¡nsson, firstname.lastname@example.org.