Five families are described in which autopsy-confirmed presenile Alzheimer's disease (AD) has occurred in men and women over multiple generations consistent with autosomal dominant inheritance. All 5 families are descendants of a group of immigrants known as the Volga Germans who came to the United States between 1870 and 1920. Their ancestors moved from Germany to the southern Volga region of Russia in the 1760s. All 5 American families are descendants of persons originally living in two small adjacent Volga German villages and share several surnames known to have been present in the census records of those villages. Although a single affected common ancestor cannot be identified, it is likely that the AD in these families represents an autosomal dominant gene inherited from one ancestor (the founder effect). This information is of importance in the genetic study of AD in these families because it greatly increases the probability of genetic homogeneity. There are more than 300,000 American descendants of the Volga Germans, and the prevalence of AD has never been studied in this population.