A case-control study of Alzheimer's disease (AD) was conducted to evaluate the roles of family history of dementing disease and AD, and medical conditions, particularly those related to the viral and immune hypotheses in AD. One hundred and eighty-three clinically diagnosed patients were identified between 1980 and 1985 through the Geriatric and Family Services clinic at the University of Washington hospital, and 18 patients were identified during the same time period through the Veteran's Administration Medical Center in Seattle, Washington. Telephone interviews were conducted with spouses (87.7%) and children (12.3%) of the patients. Each patient was matched to a friend or nonblood relative of the patient by age, sex, and the relationship between the patient and his or her respondent, for a total of 130 matched pairs. The odds ratio for AD was elevated if a first-degree relative was reported to have a history of dementia (odds ratio, 2.21; 95% confidence interval, 1.17 to 4.18). This risk was especially elevated if a sibling was affected (odds ratio, 4.04; 95% confidence interval, 1.37 to 11.90); the highest risk to the proband was observed if a sibling had presumed AD (odds ratio, 5.92; 95% confidence interval, 1.59 to 21.96). The risk increased with increasing numbers of first-degree family members affected for both a history of dementia and presumed AD. We found little difference between patients and control subjects with respect to the age of the mother or father at the time of the subject's birth. No statistically significant differences were observed between the two groups with regard to conditions that might support the immune and viral hypothesis in AD, smoking, or alcohol problems.