This paper reviews the available data on the relationships between serotonin, impulsivity, & suicide. It is concluded that low CSF-5-HIAA levels in psychiatric patients (depression, personality disorder, schizophrenia) & possibly also in supposedly healthy persons, increases the risk of suicide considerably. Other biological risk factors may also play a role. This trait marker is found in different kinds of impulsive behaviours, indicating a marker of vulnerability especially in crisis situations. Low output or low stability of the 5-HT system in the central nervous system seems to increase impulsivity. (57 refs.)