We assessed the family history of dementia and Parkinson's disease in 198 Dutch patients with Alzheimer's disease diagnosed before the age of 70 years, and in 198 age- and sex-matched healthy population controls. Of the Alzheimer patients, 48% had at least 1 1st-degree relative with dementia, compared with 19% of the controls. There were 24 patients and 1 control with 2 or more 1st-degree relatives with dementia, yielding a relative risk of 40.0. More Alzheimer patients than controls had a 1st-degree relative with Parkinson's disease, with a relative risk of 2.9. This study strongly confirms earlier findings of familial aggregation of Alzheimer's disease and provides evidence for familial aggregation of Alzheimer's disease with Parkinson's disease. The latter may point at a joint etiology of these diseases.