The authors have previously reported mild improvement of behavior and cognition in a group of 17 nondepressed patients with dementia of the Alzheimer type (DAT) treated with two doses of L-deprenyl (10 and 40 mg/day). Seven of these patients subsequently received double-blind, placebo-controlled treatment with tranylcypromine. The patients experienced significant side effects, particularly orthostatic hypotension without compensatory pulse increase, during tranylcypromine treatment. These effects were more severe than those occurring during L-deprenyl treatment, and they occurred at substantially lower doses (mean dose, 16 mg/day). These data support the role of the inhibition of monoamine oxidase type A in the development of orthostatic hypotension in patients treated with monoamine oxidase inhibitors. They also raise the question of whether the observed sensitivity to monoamine oxidase inhibition is a function of age or disease. In either case, the greater toxicity of tranylcypromine makes inferences difficult regarding the relative efficacies of these drugs in treating patients with dementia of the Alzheimer type and may limit the potential usefulness of tranylcypromine in ameliorating some symptoms of this disease.