Low concentrations of the serotonin metabolite 5-hydroxyindoleacetic acid (5-HIAA) in cerebrospinal fluid (CSF) are associated with suicidal behaviour in patients with depressive illness, but studies of the relation between CSF 5-HIAA and suicide in schizophrenia have been inconclusive and have not included long-term follow-up. In a prospective study, we measured 5-HIAA in CSF taken from 30 schizophrenic patients in a drug-free state, and followed these patients for 11 years. 10 patients made suicide attempts during follow-up. Suicide attempters had significantly lower concentrations of CSF 5-HIAA at initial evaluation than non-attempters (mean [SE] 6.7 [2.2] vs 23.6 [5.6] ng/ml, p < 0.05). Our findings provide further evidence of the relation between serotoninergic dysfunction and suicide, and suggest a role for drugs with serotoninergic effects in schizophrenia.