Lactic acidosis (LA) associated with hematologic malignancies is uncommon, life-threatening, and generally occurs in adults. Its pathogenesis is poorly understood. This is a case report of LA due to leukemic transformation that occurred in a patient with non-Hodgkin's lymphoma (NHL). A 24-year-old man with NHL was admitted to the hospital with dyspnea. Venous blood gas analysis revealed metabolic acidosis (pH 7.05; HCO3 6 mEq/L; BE 22 mmol/L; anion gap 28 mEq/L); the patient had an elevated plasma lactate concentration (12 mmol/L) and low glucose concentration (38 mg/dL). There was no reason other than leukemia-such as infection, circulatory failure, or drug use-for the development of severe LA. This case report shows that in patients with NHL, leukemic transformation may give rise to LA.