Bipolar disorder is associated with increased risk for dementia. We compared the prevalence of Alzheimer's disease between 66 elderly euthymic patients with bipolar disorder who were on chronic lithium therapy and 48 similar patients without recent lithium therapy. The prevalence of dementia in the whole sample was 19% v. 7% in an age-comparable population. Alzheimer's disease was diagnosed in 3 patients (5%) on lithium and in 16 patients (33%) who were not on lithium (P<0.001). Our case-control data suggest that lithium treatment reduced the prevalence of Alzheimer's disease in patients with bipolar disorder to levels in the general elderly population. This is in accordance with reports that lithium inhibits crucial processes in the pathogenesis of Alzheimer's disease.