The relation between head trauma and Alzheimer's disease was one of four major risk factors explored in a case-control study of 130 matched pairs; cases were clinically diagnosed between January 1980 and June 1985 at two geriatric psychiatric clinics in Seattle, Washington, and controls were friends or nonblood relatives of the cases. Subjects were matched by age, sex, and relationship between the case and his or her surrogate respondent. Head injuries which resulted in a loss of consciousness or which caused the subject to seek medical care were documented by means of interviews with surrogate respondents. A history of head injury was recorded for 24% of the cases and 8.5% of the controls, yielding an odds ratio of 3.5 (95% confidence interval 1.5-8.3) in conditional logistic regression analysis adjusted for age at onset of disease symptoms and family history of Alzheimer's disease. The estimated risk of Alzheimer's disease increased as the time between the last head trauma event and the onset of disease symptoms diminished (p = 0.002). This trend remained statistically significant (p = 0.006) when head injuries which occurred within 5 years of onset of the disease were excluded from the analysis. There was some difference between cases and controls for the average duration of unconsciousness in events accompanied by such a loss, but this was not statistically significant. The two groups were also similar in the circumstances surrounding the injuries and in the frequency of alcohol problems. This is the third case-control study to find a statistically significant association between head trauma and Alzheimer's disease.