Using a liberal criterion, a conservative probability-based criterion, and a criterion for autosomal dominant inheritance, we classified 36%, 13.5%, and 6.4% of 311 patients, respectively, as having familial Alzheimer's disease (FAD). The mean age of onset was over 70 years for all three categories of FAD. FAD and sporadic Alzheimer's disease (SAD) cases did not differ in clinical features, incidence of risk factors for dementia, or MRI or PET features. We observed earlier age of onset of AD to be related positively to longer duration of disease. Except for the autosomal dominant AD group, there was an earlier age of onset in FAD probands. The inheritance of AD from mothers was from 1.7 to 3.6 times more frequent than from fathers. Among SAD patients only, we found a preponderance of women, who were more frequently affected than would be expected from the male/female ratio in the general population of the same average age. Language performance tended to be less affected in FAD than in SAD patients, contrary to some previous reports.