The Mini-Mental State (MMSE) was administered to 2302 general practice patients aged 75 yr and over. Those scoring 23 or less and a sample of those scoring 24 or 25 were selected for further investigation using the Cambridge Mental Disorders of the Elderly Examination. Inter-observer reliability was high with a mean kappa value of 0.97. Eighty-six per cent of respondents judged to have organic mental disorders scored 23 or less on the MMSE and 92% of those judged to be cognitively intact scored 24 or more. However, only 55% of respondents who scored 23 or less were demented or delirious while a number of relatively well educated, mildly demented subjects scored 24 and 25. The customary cut-off point of 23/24 may need to be revised in future community studies. MMSE scores cannot be used to make even tentative psychiatric diagnoses; more detailed investigation of low scorers is essential.