Anaerobic glycolysis produces lactate, ATP, and water but there is no net change in the number of hydrogen ions: it does not produce lactic acid. The acidosis usually associated with hyperlactataemia is caused by hydrolysis of the ATP, with release of hydrogen ions. By contrast, ATP turnover by aerobic mechanisms is not acidifying because the released hydrogen ion is reutilised as more ATP is formed. Gluconeogenesis from lactate does not utilise hydrogen ions directly--in fact, it produces them. The associated net H+ utilisation is caused by the aerobic generation of the ATP and GTP required to drive glycolysis in reverse. It is suggested that only by understanding these important biochemical facts can the clinician found his diagnosis and treatment on a firm, rational basis.