A 24-year-old man was admitted to the University Hospital showing severe dehydration that might have been the result of medicine-induced gastritis. Wernicke's encephalopathy was suspected in this patient. On admission, primary position upbeat nystagmus (PPUN) was found. The patient showed tongue fasciculation and loss of gag reflex, which, however, soon returned to normal. Electronystagmographic (ENG) findings were suggestive of lesions in the brainstem. Based on neurological signs and symptoms, we concluded caudal brainstem lesions might be a possible site responsible for the upbeat nystagmus. This nystagmus, however, was later found to be reversed to downbeat nystagmus. This is considered to have been due to predominant differences in vertical velocity which are induced by gravity.