Cerebrospinal-fluid (CSF) corticotropin-releasing hormone, somatostatin, and thyrotropin-releasing hormone were measured by specific radioimmunoassays in 257 hospitalized psychiatry patients suffering from dementia disorders (n = 85), schizophrenia (n = 104), and mood and anxiety disorders (n = 39). Neurological controls (n = 29) were also investigated. Since there were large overlaps of the peptide levels across the nosological groups we subjected the dataset to a three-dimensional normal mixture distribution analysis. We obtained four biochemically separable clusters. Dementia disorders, but not the others, were heterogeneously distributed in these clusters but after eliminating the effects of age and illness duration this difference disappeared. No single clinical, psychological, or background variable emerged as a prominent correlate of the neuropeptide clusters. It is concluded that although CSF neuropeptide concentrations in psychiatric patient populations appear to be separable into distinct, normally distributed subgroups this distinction does not coincide with present nosological classifications.