Two cases of severe lactic acidosis induced by total parenteral nutrition (TPN) are reported. Both cases were admitted to our department for the surgical treatment of advanced gastric cancer, and subsequently underwent TPN because of poor nutritional status. Following the initiation of TPN, both patients went into an unstable circulatory state following shock. Both cases showed signs of peritonitis, suggestive of an intraabdominal abscess, and subsequently underwent an emergency laparotomy to explore the origin of the lactic acidosis. There was, however, no apparent infectious focus which would lead to severe metabolic acidosis in the abdominal cavity. One case died of irreversible shock. The other case similarly exhibited a deteriorated cardiovascular state but promptly responded to the administration of thiamine and was resuscitated. The blood thiamine level was low in both individuals, and the two patients were subsequently diagnosed as having thiamine deficiency-induced lactic acidosis.