The clinical course and metabolic disturbances in three patients with acute leukemia and severe lactic acidosis (lactic acid concentrations 11.2, 17.0 and 21.0 mmol/l) are described. Circulatory failure could be ruled out as a possible cause of elevated lactic acid. Clinical findings included somnolence, hyperventilation and diffuse abdominal pain. In patients with malignant disease, a number of factors may contribute to elevated lactic acid levels. However, in our cases the excessive lactic acidosis was due to increased production of lactic acid by the leukemic cells, together with impaired hepatic metabolization. The diminished hepatic gluconeogenesis is also documented by a severe hypoglycemia in our patients. The essential step in treatment is early diagnosis of this syndrome and prompt initiation of cytotoxic medication.