Concentrations of 5-hydroxyindoleacetic acid (5-HIAA) in the lumbar CSF were measured in a group of suicidal schizophrenic patients and in a matched group of nonsuicidal schizophrenic patients. The suicidal group had a significantly lower level. This finding is consistent with reports of low levels of CSF 5-HIAA in suicidal patients with other psychiatric diagnoses and suggests that low CSF 5-HIAA may be a generalized marker of aggressive behavior against the self and others.