Background. The South African Stress and Health (SASH) study is the first large-scale population-based study of common mental disorders in the country. This paper provides data on the 12-month and lifetime prevalence of these conditions. Methods. Data from a nationally representative sample of 4 351 adults were analysed. Mental disorders were assessed with the Composite International Diagnostic Interview (CIDI). An extensive survey questionnaire detailed contextual and socio-demographic factors, onset and course of mental disorders, and risk factors. Simple weighted cross-tabulation methods were used to estimate prevalence, and logistic regression analysis was used to study correlates of 12-month and lifetime prevalence. Results. The lifetime prevalence for any disorder was 30.3%, and the most prevalent 12-month and lifetime disorders were the anxiety disorders. The Western Cape had the highest 12-month and lifetime prevalence rates, and the lowest rates were in the Northern Cape. Conclusions. The SASH study shows relatively high 12-month and lifetime prevalence rates. These findings have significant implications for planning mental health services. S Afr Med J 2009; 99: 339-344.