There are several lines of evidence to suggest that serotonergic neurons in the brain are detrimentally affected by chronic alcohol consumption. The present study aims to quantify pathological changes in brainstem regions containing serotonergic neurons in chronic alcoholics compared to age-matched non-alcoholic controls. An antibody specific for tryp-tophan hydroxylase was used to immunohistochemically demonstrate serotonergic neurons in serial sections of postmortem brainstem. The cases analyzed were divided into four groups on the basis of their clinical and pathological presentation; chronic alcoholics with Wernicke's encephalopathy, chronic alcoholics with additional Korsakoff's psychosis, non-alcoholic controls, and a single chronic alcoholic without neurological complications. There was an overall reduction in the number of serotonergic neurons in all alcoholic cases when compared with controls. All brainstem regions were affected, but the largest neuronal loss was found in areas of the medullary and caudal pontine reticular formation (reduced by 80–90%). Alcoholics with Korsakoff's psychosis did not differ in the amount or extent of pathology from the other alcoholic cases analyzed. The data indicate that significant numbers of serotonergic neurons degenerate in chronic alcoholics. Such a loss is likely to have significant clinical consequences.