Lactic acidosis is a rare but potentially fatal complication of acute leukemia. The terminal phase of acute leukemia in a 22-year-old woman, heralded by lactic acidosis, is described in order to focus attention on the occurrence of this metabolic disturbance in the absence of overt manifestations of acute leukemia. As reported in other cases, the lactic acidosis in our patient responded poorly to bicarbonate therapy but dramatically to antileukemic chemotherapy. The pathogenesis of the lactic acidosis is unclear but it may be the result of an undefined 'paraneoplastic' metabolic disturbance induced by the leukemic cells.