Disorders caused by defects in the body’s metabolism, often due to a deficiency of a dietary factor or an enzyme. Enzymes are necessary for the normal function of the body’s numerous metabolic processes; they ensure that the level of metabolic products is not too high by converting them to simple, harmless products or to substances which can easily be excreted via the liver or the kidneys. Because specific enzymes are necessary for each chemical conversion, it follows that shortage of even one enzyme can cause an accumulation of certain materials. Various organs are affected, according to whether the disturbances concern the conversion of amino acids, carbohydrates or fats. Phenylketonuria is the best-known example of an inborn error of metabolism. The amino acid phenylalanine accumulates in the brain, causing retarded mental development. Glucose, or blood sugar, is the most important source of body energy; it can be stored in the form of glycogen. If a patient has a deficiency of the enzyme needed to break down glycogen, the glucose cannot be released and excessive glycogen accumulates, causing disorders in the function of muscles, heart, liver and brain. Fat metabolism can also be disturbed, as in Tay-Sachs’ disease, in which the fatty lining of nerve cells decays, and which in its severe form causes an early death. If the condition is present in the family, healthy carriers can be traced by a simple test. Genetic counselling, possibly combined with tests during pregnancy, is then very important in order to prevent the birth of an affected child. Incidentally prenatal tests of this kind are of value in almost all inborn errors of metabolism. Abnormal metabolic reactions show clearly in the child’s cells in the amniotic fluid. Screening at the earliest possible age is important as well as prevention in the case of inborn errors of metabolism. Thus new-born infants are tested for phenylketonuria. Early discovery of enzyme deficiencies means that treatment can be started at an early stage. Sometimes all that is needed is to follow a suitable diet to remove substances which could cause problems of metabolism. In many cases physical and mental development can proceed normally if such a diet is followed.