Concentrations of somatostatin-like immunoreactivity (SLI) in CSF were reduced in Alzheimer's disease (AD) and multi-infarct dementia (p less than 0.01), but not in normal-pressure hydrocephalus, Parkinson's disease, and Huntington's disease. This suggests that reduced SLI content in AD cerebral cortex is reflected in CSF. Chromatographic characterization of CSF SLI showed no differences between AD and controls. Concentrations of SLI in AD patients overlapped those in both normal subjects and patients with multi-infarct dementia, so that changes in CSF SLI have no diagnostic specificity.