Serotonin (5-HT), its precursor 5-hydroxytryptophan (5-HTP), and its major metabolite 5-hydroxyindoleacetic acid (5-HIAA) were measured in the cerebrospinal fluid (CSF) of 14 patients with dementia of the Alzheimer type (DAT) and in nine controls by high-performance liquid chromatography with a novel multisensor coulometric detection system. Concentrations of both 5-HT and 5-HIAA detected by this system were lower than the concentrations obtained using conventional amperometric detection. This difference was caused by coelution of compounds that could be resolved from 5-HT and 5-HIAA by the multisensor coulometric system. One of the coelution compounds, observed in DAT but not in control CSF, behaved like a partially oxidized 5-HT. A compound behaving like partially oxidized 5-HTP was also observed in DAT CSF. Concentrations of 5-HTP, 5-HT, and 5-HIAA were lower in DAT CSF than in a corresponding fraction of control CSF. These results indicate involvement of the serotoninergic system in DAT and might lead to development of a diagnostic test for DAT.