We report the case of a leukemic child treated with chemotherapy and parenteral nutrition for three weeks, who developed a severe lactic acidosis. Clinical features included both digestive and neurological disorders associated with a moderate cardiovascular collapse. After elimination of a toxic, a neoplastic or a septic cause, a thiamin (or vitamin B1) deficiency was suspected because of the lack of vitamin supply to parenteral nutrition. Intravenous administration of thiamin rapidly controlled lactic and clinical features. The diagnosis was confirmed by a low plasmatic concentration of thiamin. Thiamin deficiency must be suspected in case of severe lactic acidosis during parenteral nutrition and systematically prevented by supply of vitamins.