To test the hypothesis that primary degenerative dementia of the Alzheimer type (PDD-AT) may increase the likelihood of expression of a lifetime vulnerability to the development of depression, the authors compared the premorbid rates of major depression in psychiatric inpatients with dementia, with or without a concurrent syndrome of depression. A premorbid history of major depression was four times more common in patients with the depressive syndrome of PDD-AT than in PDD-AT patients without depression. The authors discuss the significance of these findings for pathophysiologic models and estimates of comorbidity of depression in PDD-AT.