We report the case of a patient with severe hyperemesis gravidarum. During parenteral nutrition the patient developed threatening thiamine deficiency. Wernicke's encephalopathy and tachycardia were predominant in the clinical course. Severe lactic acidosis and prolonged prothrombin time were the leading laboratory features. Thiamine deficiency as a cause of severe metabolic acidosis or coma can be easily ruled out or validated by intravenous thiamine administration.