Cerebrospinal fluid somatostatin-like immunoreactivity (CSF SLI) was determined for elderly delirious patients during the acute stage and after one-year follow-up. The SLI levels were compared with age-equivalent controls. For the group as a whole, and also when the group was subdivided according to the severity of cognitive decline at the acute stage, type of delirium, or the central nervous system disease, delirious patients showed significant reduction of SLI as compared with the controls. In the follow-up, we observed a further reduction of CSF SLI together with significant correlations in the second and third samples between SLI levels and Mini-Mental State Examination score. Our results suggest a role for somatostatinergic dysfunction in the genesis of some symptoms of delirium. This dysfunction may be a common phenomenon in various forms of delirium and dementia.