Characteristics of familial aggregation of Alzheimer's Disease were studied in 92 families ascertained through a clinically diagnosed proband with an onset below age 60 years. In each family data were systematically collected on the sibships of the proband, of his father, and of his mother. A total of 926 relatives were included and 81% of the living relatives (i.e., 251 individuals) were directly examined. The estimated cumulative risk among first degree relatives was equal to 35% by age 89 years (95% confidence interval 22 to 47%). This result does not support the hypothesis that an autosomal dominant gene, fully penetrant by age 90 years, is segregating within all these pedigrees. Despite the fact that all probands were selected for an onset before age 60 years it was shown that two types of families could be delineated with respect to age at onset among affected relatives: all secondary cases with an onset below age 60 years were contributed by a particular group of families (type 1 families), whereas all secondary cases with an onset after age 60 years were contributed by another group of families (type 2 families). Although genetic interpretation of these findings is not straightforward, they support the hypothesis of etiologic heterogeneity in the determination of early-onset Alzheimer's disease.