• Cerebrospinal fluid concentrations of the monoamine metabolites 5-hydroxyindoleacetic acid (5-HIAA), homovanillic acid (HVA), and 3-methoxy-4-hydroxyphenyl glycol (MHPG) were measured in 30 psychiatric patients who had attempted suicide and 45 healthy volunteers. The suicide attempters had a significantly lower CSF 5-HIAA level than the controls, especially those who had made more violent attempts. After adjustment for differences in body height and age between controls and patients, the difference in 5-HIAA level became even more marked. Concentrations of 5-HIAA also were lower than normal in suicidal patients who were not diagnosed as depressed at the time of lumbar puncture, while HVA levels were lowered only in the depressives. A follow-up study of these and 89 more patients (depressed and/or suicidal) revealed a 20% mortality by suicide within a year after lumbar puncture in patients with a CSF 5-HIAA level below the median.