Objective: This study examined the frequency of major and minor depression in Alzheimer's disease and determined whether these types of depression have a different functional and psychopathological impact and whether there is a change in the prevalence of major and minor depression throughout the stages of Alzheimer's disease.Method: A consecutive series of 670 patients with probable Alzheimer's disease were assessed with the Structured Clinical Interview for DSM-IV; specific instruments to rate the presence and severity of depression, anxiety, apathy, irritability, delusions, pathological affective crying, performance of activities of daily living, and social functioning; and a standardized neuropsychological evaluation. Diagnoses of major and minor depression were generated from DSM-IV criteria.Results: Twenty-six percent of the patients had major depression, 26% had minor depression, and 48% were not depressed. Major depression was significantly associated with sad mood in all three stages of the illness, although this association dropped significantly for minor depression in severe Alzheimer's disease. Both major and minor depression were significantly associated with more severe psychopathology, functional impairments, and social dysfunction. Depressive symptoms that most strongly discriminated between Alzheimer's disease patients with and without sad mood were guilty ideation, suicidal ideation, loss of energy, insomnia, weight loss, psychomotor retardation/agitation, poor concentration, and loss of interest.Conclusions: Our study demonstrates that DSM-IV criteria for major and minor depression identify clinically relevant syndromes of depression in Alzheimer's disease, mild levels of depression can produce significant functional impairment, and the severity of psychopathological and neurological impairments increases with increasing severity of depression.