Two young females with severe morbid obesity presented with Wernicke's syndrome after Roux-en-Y gastro-jejunum bypass had been performed. The first patient had recurrent vomiting and dyplopia two months post-surgery. Physical examination indicated bilateral ophthalmoparesia with conserved convergence and ataxia. The second patient had frequent vomiting episodes over the previous three months together with lower limb hypotonia, myoclonia and generalised tonicoclonic seizures on two occasions within one year of surgery. In both cases routine blood test, ion levels (sodium, potassium, calcium, phosphates), electroencephalogram and CT scan were normal. Thiamine therapy was instigated on the basis of clinical intuition and the first patient achieved complete remission within 24 hours while the second improved gradually in that two years later only mild lower limb hypotonia and a slight cognitive deficit remains. Erythrocyte transketolase activity determinations were abnormal on two separate occasions for this second patient. Vitamin B1 determinations were not available for the first patient. In conclusion, the restriction in energy intake and the persistent vomiting together with malabsorption induced by the surgical intervention could explain the vitamin deficiency causing Wernicke's encephalopathy. This indicates a need for close monitoring and systematic vitamin supplementation in those patients who undergo bariatric surgery.