An autopsy performed on a 53-year-old man who had unrelieved obstruction of the small bowel due to gallstone ileus revealed morphological features that were typical of acute Wernicke's encephalopathy. The likely sequence of disease in this patient was the development of thiamine deficiency owing to the unrelieved intestinal obstruction, which resulted in the development of acute Wernicke's encephalopathy. A high clinical awareness is required for the diagnosis of acute Wernicke's encephalopathy in patients with malnutrition from any cause.