The Wernicke-Korsakoff syndrome is a neuropathological term which encompasses two clinical syndromes in thiamine-deficient alcoholics, Wernicke's encephalopathy and Korsakoff's psychosis. Wernicke's encephalopathy is characterised by eye and gait disorders and mental confusion, and can lead to the profound and permanent amnesia known as Korsakoff's psychosis. Despite this specific difference, both conditions appear to have identical neuropathology with haemorrhages and other lesions around the ventricular system. The memory deficit has been attributed to a number of brain lesions, including a recent suggestion that brain pathways utilising particular neurotransmitters are specifically damaged. To examine this, the number of chemically identified neurons in particular brain regions was quantified in patients with Wernicke's encephalopathy alone or in combination with Korsakoff's psychosis and compared with age- and sex-matched controls. Noradrenaline, a neurotransmitter thought to be involved in the process of selective attention, is localised in pathways projecting to the cortex. Our patients with either Wernicke's encephalopathy or additional Korsakoff's psychosis do not differ from controls in the distribution and number of these cells. Serotonin is another neurotransmitter that has been linked with alcohol dependency. Both patient groups have a profound loss of serotonin-containing neurons compared with controls. The loss of forebrain neurons containing acetylcholine in patients with Alzheimer's disease has implicated this neurotransmitter in the maintenance of memory functions. There was a large variation in the number of these forebrain neurons in thiamine-deficient alcoholics compared with controls. Cholinergic cell loss reflected the severity of cognitive dysfunction, but was not exclusive to patients with amnesia. Two thalamic nuclei are involved in relaying memories for storage and retrieval, the anterior and mediodorsal thalamic nuclei. While patients with Wernicke's encephalopathy often had neuronal loss in the mediodorsal nucleus, only patients with Korsakoff's psychosis had cell loss in both medial thalamic nuclei. The results suggest that cumulative lesions contribute to the amnesia seen in thiamine-deficient alcoholics, including deficits in serotonergic, cholinergic and medial thalamic pathways.