Two cases of children treated for malignant tumor and who developed a severe lacticacidosis are reported. A 2 year-old girl and a 8 year-old boy were treated with chemotherapy, irradiation and multiple surgical procedures for nephroblastoma and lymphoma respectively. These two malnourished patients, under exclusive parenteral nutrition for two weeks without vitamin intakes, suddenly developed neurological, cardiovascular and digestive symptoms, associated with cytopenia and lacticacidosis. Injection of vitamin B1 only corrected these abnormalities within a few hours, proving the role of thiamine deficiency as the cause of the symptoms.