Concentrations of somatostatin-like immunoreactivity in cerebrospinal fluid were significantly reduced in chronic pain patients compared to control patients without chronic pain. This difference was not influenced by demographic or clinical characteristics. Somatostatin has been shown to be a neurotransmitter in animal nociception; pharmacologic doses of this substance have moderated human pain. Our findings provide evidence that somatostatin may be involved in the pathogenesis of the chronic pain state.