AbstractThis paper reviews the available data on the relationships between serotonin, impulsivity and suicide. It is concluded that low CSF‐5‐HIAA levels in psychiatric patients (depression, personality disorder, schizophrenia) and possibly also in supposedly healthy persons, increases the risk of suicide considerably. Other biological risk factors, e.g. activation of the hypothalamic—pituitary axis (positive dexamethasone‐suppression test) may also play a role. This trait marker is found in different kinds of impulsive behaviours: suicide, arson, murder, alcoholism, disinhibitory personality traits and others, indicating a marker of vulnerability, especially in crisis situations. Low output or low stability of the 5‐HT system in the CNS seems to increase im