Mutations in three different genes on chromosomes 1, 14, and 21 cause autosomal dominant forms of familial Alzheimer's disease (FAD). Most result in an early-onset phenotype. However, several kindreds of Volga German ancestry have the same chromosome 1 gene mutation and demonstrate a relatively older mean age of onset and include individuals with late age of onset. In these families, the mean age of onset is 54.9 ± 8.4 years (range, 40-75 years), mean age at death is 65.9 ± 10.2 years (range, 43-88 years), and mean disease duration is 11.3 ± 4.6 years (range, 5-23 years). This contrasts with a group of 7 families with chromosome 14 mutations in which the mean age of onset is 44.8 ± 4.8 years (range, 30-55 years), mean age at death is 52.6 ± 5.7 years (range, 39-65 years), and mean disease duration is 7.6 ± 3.2 years (range, 2-17 years). (All means are significantly different in the 2 groups of families, p < 0.005.) In the chromosome 1 families, 7 persons (16%) had an age of onset at or older than 65 years and 22 (54%) survived to age 65 or older versus none in the chromosome 14 families. An example of probable nonpenetrance of disease at age 89 was also found in a chromosome 1 kindred. It is concluded that, unlike the chromosome 14 gene, mutations in the chromosome 1 FAD gene may result in individuals with a late age of onset overlapping with the more common sporadic form of the disease occurring in the general population. In light of the great variability in age of onset in persons with identical mutations, study of the genetic and environmental factors contributing to delayed onset of disease in chromosomal 1 FAD kindreds will be an important area for further investigation. Apolipoprotein E genotype may be one such factor that plays a partial role in this variability.