The incidence of severe dementia between the age of 70 and 79 years was studied in a representative urban sample. The study comprised 385 subjects at 70 and sufficient information was obtained in 94.5% in the age interval 70-75 and in 89.2% in the age interval 75-79. A case was defined by absence of symptoms of severe dementia at the beginning of the interval and by disorientation and/or severe memory impairment in psychiatric interviews at the age of 75 or 79, or the same signs noted in case records from the intervals studied. In all, 43 subjects developed severe dementia. When mortality was taken into account the annual incidence rates were 17.7 +/- 10.7/1,000 in the 5-year interval between 70 and 75 and 32.3 +/- 16.9/1,000 in the 4-year interval between 75 and 79 in men compared with 5.0 +/- 4.9/1,000 and 25.3 +/- 11.3/1,000 respectively in women. There were no significant differences between men and women with regard to the incidence rates of all severe dementias or of etiological subgroups. There was a high degree of institutionalization. Most studies from Scandinavian countries have reported lower incidence figures.