Several inherited metabolic diseases are accompanied by a greater or lesser increase in blood lactate concentration under certain metabolic conditions. These diseases are glycogenosis type I (glocuse-6-phosphate deficiency), fructose-1,6-diphosphatase deficiency, glucose-induced hyperlactate emia, idiopathic lactate acidosis. The conditions are discussed when hyperlactate emia develops. Very large increases in blood lactate concentration are found during muscular activity, lactate concentrations can be as much as 20 mmol/l under these conditions. Regarding these values, the increase in blood lactate concentration during intravenous carbohydrate infusion is minimum, even in the case of fructose infusions (1-4 mmol/l). Therapeutical measures for treatment of increased lactate concentration are discussed. A causal therapy is optimum; however, the precondition is a definite diagnosis. Besides bicarbonate infusions (or infusions of other alkalizing substances) dialysis seems to be a favourable therapy in certain cases. In future, prognosis of lactate emia should be better if the diagnostic measures and differential diagnosis are improved.