All patients attending the Clinic for Alzheimer Disease and Related Disorders have detailed family histories taken by a geneticist. To date, genetic histories are available for 446 consecutive, unrelated individuals. Of these, 151 (33.9%) are diagnosed as having "probable" (N = 141) or "autopsy-confirmed" (N = 10) Alzheimer disease according to recognized criteria. This data base represents a relatively unselected population with respect to more than one person in the family having dementia. Seventy-one of these 151 index cases (47.0%) have a positive family history of dementia, of which 8 (5.3%) may represent the familial (autosomal dominant) form of Alzheimer disease (FAD). Age-corrected empiric recurrence risks for Alzheimer disease/dementia were calculated for first-degree relatives of these 151 index cases using the Kaplan-Meier lifetable method.