BACKGROUND Wernicke's encephalopathy, a pathology caused by vitamin B1 (thiamin) deficiency, is often difficult to diagnose and can lead to severe cognitive sequels if left untreated. METHODS We report the case of a 42-year-old HIV-positive women who, four days after recurrent episodes of vomiting, developed severe dysautonomia and symptoms suggestive of Wernicke's encephalopathy. Treatment with parenteral thiamine induced dramatic improvement within a few days. CONCLUSIONS This case report highlights an unusual presentation of symptomatic thiamin deficiency associating severe dysautonomia with the classical manifestations of Wernicke's encephalopathy. As dysautonomia is frequently the earliest sign of beriberi, this case illustrates the continuum between these two diseases whose cause, symptomatic thiamin deficiency, is the same. It also draws attention to the multiple risk factors that may be associated, leading to symptomatic thiamin deficiency. This deficiency, while often overlooked, is frequent in HIV-infected patients. Finally, this case contributes to the discussion on the possible genetic polymorphism that may make a limited deficiency symptomatic.