This study reports results of a large-scale epidemiological investigation of the prevalence of mental disorder in Oslo. A random sample of Oslo residents age 18–65 years was drawn from the Norwegian National Population Register. A total of 2,066 subjects, 57.5% of the original sample, were interviewed with the Composite International Diagnostic Interview in 1994–1997. The mean age of the interviewed subjects was 39.3 years. The 12-month prevalence of all mental disorders was 32.8%, and the lifetime prevalence was 52.4%. Alcohol abuse/dependence and major depression had the highest lifetime prevalence and 12-month prevalences. All mental disorders were more prevalent in women than in men, with the exception of alcohol and drug abuse/dependence. Severe psychopathology (e.g., three or more diagnoses) was found in 14%–15% of the respondents. The lifetime and 12-month prevalences for all diagnostic categories except drug abuse/dependence were similar to those found in the United States Comorbidity Survey. Epidemiological data for Oslo show that the lifetime and 12-month prevalences of mental disorder are quite high, with alcohol abuse/dependence and major depression particularly frequent. The rates for women are higher than those for men for all diagnostic categories, except for alcohol and drug abuse/dependence.