The rare congenital lactic acidosis is a consequence of enzyme defects. The acquired form is relatively common in critically ill patients. The altered metabolism of pyruvate and the imbalance between lactate production and utilization have a central role in the pathogenesis of this disease. The physiologic compensating mechanisms are generally not sufficient for complete correction of acidosis. In most of the cases the basic disease is the one that should be treated. The correction of the acidosis must be careful, because overtreatment may worsen acidosis, or may cause severe post-treatment alkalosis.