The investigation of genetic and other causes of Alzheimer's disease requires reliable and valid methods of gathering historical data on elderly relatives. To assess the reliability of the family history method in the study of late-onset dementing illness, the authors studied families with Alzheimer's disease probands and control families. The results indicated that family history data obtained from multiple informants with standardized instruments are consistent across informants. The rate of Alzheimer's disease in the families studied was similar to rates found in previous studies.