Type B lactic acidosis is a rare condition in patients with solid tumors or hematological malignancies. Although there have been several theories to explain its mechanism, the exact cause of lactic acidosis remains to be discovered. Lactic acidosis is usually related to increased tumor burden in patients with malignancy. We experienced a case of lactic acidosis in a 39-year-old man who visited an emergency room because of dyspnea, and the cause of lactic acidosis turned out to be recurrent acute leukemia. Chemotherapy relieved the degree of lactic acidosis initially, but as the disease progressed, lactic acidosis became aggravated. Type B lactic acidosis can be a clinical presentation of acute exacerbation of acute leukemia.