A 74-year-old man with parkinsonism developed progressive cognitive and behavioral dysfunction suggesting coexistent Alzheimer's disease. The intellectual and behavioral disturbances were reversed following withdrawal of his anticholinergic antiparkinsonian medication. This case demonstrates that anticholinergic drugs used to treat parkinsonism may mimic or exacerbate the clinical signs of Alzheimer's disease and suggests that these medications should be withdrawn for all parkinsonian patients who develop significant impairments of cognition or behavior.