Lactic acidosis is a major cause of metabolic acidosis in critically ill patients. Herein we report a child with relapsed non-Hodgkin’s lymphoma admitted to the pediatric intensive care unit (PICU) with profound lactic acidosis. On admission, he was treated with fluid replacement and a vasopressor, followed by continuous veno-venous hemodiafiltration to correct acidosis. As lactic acid levels remained high despite all treatments, thiamine was added to the therapy, which did not influence metabolic status either. Lactic acidosis could only be corrected by aggressive chemotherapy during his stay in the PICU. The patient died on the 68th day of PICU admission due to underlying progressive disease. Clinicians should start aggressive chemotherapy as soon as possible in patients with a recurrence or advanced cancer who have type-B lactic acidosis.