The lifetime prevalence rates are presented for mental disorders in a random sample of people born in Iceland in 1931, interviewed at the age of 55-57 years. The diagnoses are made according to DSM-III, on the basis of the National Institute of Mental Health's diagnostic Interview Schedule (NIMH-DIS) used by trained lay interviewers. The most common diagnoses were alcohol abuse and dependence, generalized anxiety disorder, phobic disorders, dysthymic disorder and major depressive episode. Disorders more common in men were antisocial personality, alcohol abuse and alcohol dependence. Disorders more common among women were major depressive episode and generalized anxiety disorder. Alcohol abuse was more prevalent among those living in rural areas, but dependence was more prevalent in the urban area, where panic disorder is also more frequent. Widowed, separated and divorced people had most of the highest prevalences: tobacco-use disorder, alcohol abuse and dependence, dysthymia and generalized anxiety disorder. Except for a very high rate of alcohol abuse and dependence and a low rate of substance abuse disorders, the prevalence rates are similar to those obtained in North American studies using the NIMH-DIS as a survey instrument. The DSM-III criteria for alcohol abuse or dependence may be less applicable to Iceland than to North America, because of differences in what is culturally regarded as acceptable use of alcohol.