Lactic acid is generated as the end product of anaerobic metabolism of glucose and is disposed by gluconeogenesis or oxidation. Changes in the lactate pyruvate ratio are not necessarily indicative of tissue hypoxia. The plasma lactate concentration is the result of lactate production and lactate removal (hepatic and renal gluconeogenesis; oxidation by muscle, liver and kidney). Lactic acidosis is defined as a state of metabolic acidosis (arterial pH less than 7.3) due to an increase in the blood concentration of lactate (greater than 2 mEq/l). Lactic acidosis may occur with evidence of tissue hypoxemia (type A) or in its absence (type B). Lactic acidosis has been described in association with phenformin therapy, hereditary enzymatic defects, hematological malignancy, prolonged fasting, shock with or without septicemia and occasionally without any underlying disease ("idiopathic" lactic acidosis). The therapy of lactic acidosis consists of administration of sodium bicarbonate and restoration of adequate tissue perfusion; hemodialysis may be helpful to control sodium excess and possibly to remove phenformin. The effectiveness of methylene blue, glucose and insulin are not yet established.