To ascertain the frequency and distribution of new cases of dementing illness in the elderly population of Mannheim (population 308000), a survey sample of community residents of > 65 years of age was re-examined after an interval of 7–8 years, and a sample of elderly persons in long-stay care after 5–6 years. The estimated annual incidence rate for all forms of dementia, after correction for ageing of the samples, was 15·4 per 1000 persons aged > 65, made up of dementia of Alzheimer type (8·9 per 1000), vascular dementia (4·4 per 1000) and other forms (2·1 per 1000). Apart from the expected association with age > 65 years, the most powerful predictors of onset of a dementing illness were residence in long-stay care and the presence of minor cognitive deficits at the initial examination, each of which was associated with a highly significant increase in risk. In addition, a number of socio-demographic characteristics (marital status, occupational history and quality of present living accommodation) was found to be predictive, though analysis of larger samples or pooled research data would be necessary to establish their importance in this respect. The findings suggest that identification of mild cognitive impairment on screening could prove helpful in assessing risk for dementia from two to three years before the condition becomes clinically apparent.