Background: Most community surveys of the mental health of older people have used fully structured interviews administered by lay interviewers. In this study of an older Brazilian population, the authors investigated the prevalence and correlates of common mental disorders with semistructured interviews administered by a psychiatrist. Method: A two phase population survey of 392 persons aged ? 75 years were screened for depression and mental disorders using the General Health Questionnaire (GHQ-12) and Geriatric Depression Scale (GDS). Half of those scoring >= 11 on GDS and >= 4 on GHQ, and 20% of others, were evaluated in the second phase using the Schedules for Clinical Assessment in Neuropsychiatry (SCAN) to generate International Classification of Diseases, 10th Revision diagnoses. Results: The prevalence of depressive episode was high (19.2%) with no effect of gender or age. Dysthymia is also prevalent (9.0%), but half of the cases were comorbid with depression. Anxiety and somatoform disorders were comparatively infrequent. Conclusions: Structured lay-administered interviews may underestimate the prevalence of mental disorders in older people. One month, rather than one year or lifetime, prevalence may be more appropriate for the oldest-old given patchy recall of distant experiences. The very strong association between mental disorders and poor self-reported global health underlines their impact on quality of life2099-12-31