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Alzheimer's disease (AD) dementia has been assessed in 6,660 first-degree relatives of 1,049 probands 60-102 years of age and without dementia. Assessment was accomplished through family informants. Persons who have lived beyond 90 years of age without dementia are hypothesized to have greater concentrations of genetic protective factors against AD vs younger undemented people. It appears that genetic factors that contribute to reduced liability of AD over a lifetime may be more concentrated among nondemented probands aged 90 or more years and their families. Probands, those from whom a lineage is traced, were grouped by age and the cumulative survival from AD and the 10-year-age-interval hazard rates of AD were calculated in first-degree relatives. Cumulative survival from AD was significantly greater in the relatives of the oldest proband group, aged 10-102 years. Reduction in the rate of illness in the same group was more or less constant in later life.