Cerebrospinal fluid 5-hydroxyindoleacetic acid (CSF 5-HIAA) was determined for elderly delirious patients during the acute stage and after a 1-year follow-up period, and the 5-HIAA levels were compared with age-equivalent controls. As compared with the controls, the 5-HIAA levels were significantly higher at the beginning of the index admission in patients with multi-infarct dementia and patients with no apparent CNS disease. The 5-HIAA levels were also higher in the latter subgroup in the 1-year sampling, but no other differences between delirious patients and controls were observed. The one-way procedure showed no differences between the subgroup means of delirious patients when divided according to the severity of cognitive decline or type of delirium in any of the samples. The 5-HIAA levels measured during the index admission correlated with the length of life after delirium suggesting that serotonergic dysfunction may have prognostic significance in delirious patients.