Ninety-one families of Alzheimer patients were studied to determine the proportion of familial cases, to obtain pedigrees for the analysis of the mode of inheritance, and to look for clinical differences between the familial and the nonfamilial cases. The diagnosis was confirmed by autopsy in 26 cases. Thirty-nine cases (43%) were familial, which is defined as more than one case in the family. Our interpretation of the pedigree data is that Alzheimer's disease is etiologically heterogeneous: it may be genetic or sporadic. In the familial type we think that the disease is inherited as an autosomal dominant, with a wide range of age of onset within a family. In one-third of these families the gene is not expressed until over age 70. No clinical differences were found between the familial and the sporadic groups.