A nationwide epidemiologic study of presenile dementia of the Alzheimer type (PDAT) with onset through age 60 years was carried out in Israel. The Israeli National Neurologic Disease Register and clinical records of all patients discharged from hospitals between 1974 and 1983 with a neurologic or psychiatric diagnosis suggestive of dementia were reviewed. A total of 71 Jewish patients with onset of PDAT between 1974 and 1978 was ascertained. The age at onset in these patients ranged from 43 to 60 years. The median survival was 8.1 years, with slightly longer survival if onset occurred before age 55 years, even after correction for expected mortality according to age and sex. The average annual incidence rate per 100,000 population at risk was 2.4 in the population aged 40 through 60 years. Although the incidence rates were slightly greater for women, the difference between the rates by sex was not statistically significant. The age- and sex-adjusted incidence of PDAT per 100,000 population was significantly higher in those born in Europe or America (2.9) than in those born in Africa or Asia (1.4). No significant difference in survival was found between these two groups. The curve of the incidence rates by age for PDAT in Israel is continuous with that for senile dementia of the Alzheimer type collected by similar methods elsewhere, which suggests that one disease process may account for both conditions.