BACKGROUND Type-B lactic acidosis is a rare complication of solid tumors and hematological malignancies. It occurs secondary to Warburg effect, when glucose metabolism in cancer cells switches from the oxidative pathway to the glycolytic pathway. Malignant lactic acidosis is a life-threatening condition if not promptly diagnosed and treated urgently. CASE REPORT We report the case of a 58-year-old male patient who presented with severe chest pain, dyspnea, systemic symptoms, leukopenia, normocytic anemia, and severe lactic acidosis. He was admitted with a possible diagnosis of acute pericarditis and lactic acidosis. Sodium bicarbonate replacement did not improve the lactic acidosis. Liver biopsy was performed because of persistently elevated alkaline phosphatase and gamma-glutamyl transferase; the biopsy showed atypical lymphoblasts and bone marrow biopsy confirmed the diagnosis of precursor B acute lymphoblastic leukemia. Lactic acidosis normalized after initiation of chemotherapy. CONCLUSIONS Cancer, particularly hematological malignancy, should be considered as an etiology and differential diagnosis of type-B lactic acidosis. Prompt recognition and urgent initiation of specific therapy to control the underlying malignancy are critical to manage this serious metabolic complication.