First-degree relatives of patients with Alzheimer's disease (AD) are at greater risk for dementia when compared with the relatives of their healthy peers, but not when compared with the relatives of patients with Parkinson's disease. This may indicate that the risk of dementia in these relatives is not specific to AD or that these studies are biased. We obtained a family history and vital status information on each first-degree relative of patients attending a clinic and in a group of recruited healthy elderly subjects. Patients formed two groups: probable AD and other forms of dementia or cognitive disorders without dementia. The odds of dementia in first-degree relatives did not differ between patient groups. The odds of dementia in relatives of patients with probable AD or other forms of dementia was six times that in the relatives of the healthy elderly subjects. The cumulative incidence of dementia increased with age in the first-degree relatives of all subjects. Approximately 50% of the first-degree relatives of patients with AD were demented by age 91 years, but almost the same number of the other patient group's relatives were demented as well. That figure was never reached in the healthy elderly subject's relatives. Because the risk of dementia in first-degree relatives of patients with AD was similar to that for patients with other disorders, we cannot exclude the possibility that this is the result of selection and information biases. Our investigation implies that the increased risk of dementia may not be specific to relatives of patients with AD; the risk may also be increased in first-degree relatives of patients with other neurologic disorders.