Objective: To report a case of severe lactic acidosis, encephalopathy, and hypotension in a patient receiving total parenteral nutrition without multivitamin supplementation and the dramatic response to intravenous administration of thiamine. Case Summary: A 15-year-old African-American girl undergoing treatment for chronic myelogenous leukemia with allogenic bone marrow transplantation and requiring total parenteral nutrition was admitted to our intensive care unit for management of life-threatening lactic acidosis, encephalopathy, and hemodynamic instability. Because of the manufacturing shortage of parenteral multivitamin preparations, oral multivitamins were prescribed; however, the patient was unable to swallow the oral multivitamins because of oral mucositis. Intravenous administration of thiamine promptly reversed the profound metabolic, neurologic, and hemodynamic abnormalities. Discussion: Carbohydrate loading associated with inadequate thiamine intake may result in potentially lethal metabolic, neurologic, and hemodynamic alterations. Prompt intravenous administration of thiamine can rapidly reverse metabolic acidosis, encephalopathy, and shock in such situations. Conclusions: Adequate administration of multivitamins must be ensured in all patients receiving total parenteral nutrition.