Cerebrospinal fluid concentrations of 5-hydroxyindoleacetic acid (5-HIAA), homovanillic acid (HVA) & 3-methoxy-4-hydroxyphenyl glycol (MHPG) were measured in 30 psychiatric patients who attempted suicide & 45 healthy volunteers. The suicide attempters had a significantly lower CSF 5-HIAA than the controls, especially those who had made more violent attempts. 5-HIAA was also lower than normal in suicidal patients who were not diagnosed as depressed, while HVA was lower only in the depressives.