Delirium is a common complication of dementia and may produce considerable morbidity. In addition to psychotic symptoms such as hallucinations and delusions, delirium may produce considerable agitation, which may be refractory to conventional medications such as antipsychotics and benzodiazepines. The main approach to delirium is to treat any underlying medical problem that could cause the delirium. However, delirium is not always reversible, and there is no specific treatment for persistent delirium. The authors present a case of delirium complicating a preexisting dementia that resolved rapidly following initiation of the cholinesterase inhibitor donepezil, suggesting that cholinergic dysfunction may have played a role in the etiology of this patient's delirium. Future research needs to be directed at the issue of cholinergic activity in delirium through monitoring of serum anticholinergic activity and its response to procholinergic therapy.