We describe a women of 22 years of age who had had a vertical gastroplasty (as treatment for morbid obesity). She was admitted to hospital with a 4 week history of nausea and vomiting of food. Treatment with intravenous dextrose, without vitamin supplements was started. One week later she complained of diplopia, paresthesia and weakness of the limbs. All investigations proved to be normal. A deficiency state was suspected, probably Wernicke's encephalopathy, although no alterations were seen in her mental state. We started treatment with high doses of parenteral thiamine, other vitamins and a suitable diet. The treatment was followed by complete recovery. Few neurological complications have been described in association with vertical gastroplasty. The commonest are polyneuropathies. The probable deficiency origin of these is considered. We emphasise the importance of vitamin supplements following the surgical treatment of morbid obesity to avoid the development of deficiency states. Wernicke's encephalopathy is due to thiamine deficiency. It may be associated with any type of malnutrition, not only with chronic alcoholism. The full clinical triad which is diagnostic of this condition is only present in one third of the cases. When the condition is suspected on clinical grounds treatment should be started early to avoid the occurrence of irreversible secuelae.