Alzheimer's disease (AD) patients frequently manifest delusions, and the cholinergic deficiency of AD may contribute to this aspect of the psychopathology of the disorder. In a double-blind, crossover study involving two patients, we compared the antidelusional efficacy of physostigmine, an acetylcholinesterase inhibitor, with haloperidol, a widely used neuroleptic agent. Physostigmine ameliorated the delusions and produced fewer side effects. These preliminary observations suggest that the cholinergic deficiency contributes to the occurrence of delusions in AD and cholinergic therapy may have a role in the treatment of the delusional symptoms.