Cerebrospinal fluid acetylcholinesterase (CSF AChE) was determined for elderly delirious patients during the acute stage and after a 1- and 4-year follow-up periods, and the AChE levels were compared with age-equivalent controls. The AChE levels measured during the index admission correlated with the length of life after delirium, suggesting that cholinergic dysfunction may have prognostic significance in delirious patients. Although the CSF AChE concentrations measured during the index admission were in the same range as in controls, we observed a declining trend in patients with various structural brain diseases during the follow-up period. The decreasing levels may reflect the progression of the underlying dementia in these patients.