Prevalence rates for psychiatric disorders in the elderly are presented from the initial cross-sectional stage of a longitudinal community study of the incidence of dementia in the city of Liverpool. Together with five other centres in the UK the MRC-ALPHA project forms part of the MRC multicentre incidence study of dementia and cognitive decline. An age- and sex-stratified random sample of 5222 subjects aged > or = 65 was interviewed at home using the Geriatric Mental State-AGECAT package to provide computer diagnoses. The overall age-standardized prevalence rates for organic disorder (4.7%) depressive illness (10.0%) and the neuroses (2.5%) are consistent with levels found in previous smaller studies that have used GMS-AGECAT. Each of these diagnoses is more common in females than males. A rise in organic disorder with age is confirmed as continuing into the oldest age groups for both sexes. An apparent decline with age observed for depression and neurosis diagnoses disappears when organic cases are excluded from the analysis.