Genetic metabolic errors are up to now mainly treated at the level of the non-metabolized substrate (diet, advanced elimination, blocking of the syntheses) or of the deficient metabolic product (e.g. substitution of hormones). In some defects at the level of the enzymatic protein an activation by administration of cofactors (vitamin-dependent defects), an induction of the syntheses of enzymes with residual activity, and the substitution by transplantation of different organs is partly possible. Studies about the direct substitution of highly purified enzymes are suggested to be successful in the near future. By this treatment the metabolic error may be compensated and clinical signs are prevented. The disease, however, is not curable. At present it is not possible to predict, whether or not it will be possible some day to cure metabolic errors in man by means of genetic engineering.