A review of 27 research reports on the cerebrospinal fluid levels of neurotransmitter metabolites involving 1202 psychiatric patients found strong evidence for the involvement of the serotonin system in suicidal behavior. Attempted suicides, especially those using violent methods, had lower levels of CSF 5-HIAA as compared to psychiatric controls, and those making subsequent suicidal actions also had lower levels of CSF 5-HIAA. The meta-analysis permitted more reliable conclusions to be drawn than did each individual study alone.