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Depression is often accompanied by other disorders including Alzheimer's disease and Parkinson's disease. We studied the familial aggregation of these disorders in order to examine the possibility of a shared genetic origin. In a population-based study of 6596 subjects, we studied the association of self-reported depression, which required treatment by a psychiatrist, to family history of psychiatric disease, dementia, and Parkinson's disease. A family history of psychiatric disease was significantly associated with overall depression as well as with unipolar (n = 303 patients) and bipolar (n = 27 patients) depression. The risk of unipolar depression was associated with the presence of two or more demented individuals among their first degree relatives (e.g. parents, siblings and children). Since there was no evidence for familial aggregation in subjects with only one demented relative, our study suggests that unipolar depression may be associated specifically to a strongly familial, form of dementia. The risk of bipolar depression was increased for those with one or more relatives with dementia and, perhaps, for those with relatives with Parkinson's disease. The familial aggregation of depression with dementia and perhaps Parkinson's disease suggests that there may be shared susceptibility gene(s) underlying these diseases. Our study indicates further that there may be differences in the genetic etiology between unipolar and bipolar depression.