Research regarding the possible association between Alzheimer's disease and a history of depression has been inconclusive. Using a case-control design, we assessed the strength of the association between reported history of depression and onset of Alzheimer's disease. We enrolled probable Alzheimer's disease cases (N = 294), who were ascertained and diagnosed by our Alzheimer's Disease Patient Registry, and randomly selected nondemented controls (N = 300) of similar age and gender from the same base population. The mean age (for cases) was 78.5 years. Informants provided data regarding history of depression. "Treated depression" was defined as depression for which a physician/psychologist consultation, medication, or hospitalization had occurred. Restricting treated depression to exclude primary loss or grief reactions, we found a modest association with Alzheimer's disease [odds ratio (OR) = 1.8; 95% confidence interval (CI) = 0.9-3.5] after adjusting for gender, age, education, and type of informant. When these data were stratified by depression onset year, we observed an odds ratio of 2.0 (95% CI = 0.9-4.6) for depression occurring more than 10 years before the onset of dementia symptoms, and an OR of 0.9 (95% CI = 0.2-3.0) for depression onset within 10 years of the onset of dementia symptoms. Thus, depressive episodes occurring well before dementia symptom onset appear to increase the risk of Alzheimer's disease.