Idiopathic neonatal jaundice derives from an initial insufficiency of all processes which metabolize hydrophobic bilirubin into diglucuronide excretable in bile. The term 'neonatal hyperbilirubinemia' should only be used when there is a potential risk of bilirubin intoxication. Thus, the concept hyperbilirubinemia is not necessarily linked to the exceeding of a certain threshold value, but rather to the maturity of the child and its clinical condition. In this sense, hyperbilirubinemia is, therefore, always a syndrome requiring treatment. Besides substitution transfusion, which is highly effective per se, but risky and costly, enzyme induction (e.g., by administration of phenobarbital) represents an elegant causal therapy; however, because of its slow onset of action, it has to be given prophylactically to almost all newborns. In addition, this method requires a general induction of all microsomal enzyme systems, and is hence a major intervention in the process of maturation of the neonatal enzyme systems. Therefore, phototherapy must be regarded as the treatment of choice in cases of idiopathic neonatal hyperbilirubinemia. It leads to a bypassing of the hepatic enzyme insufficiency in that by interaction between light with a wavelength of around 460 nm and the bilirubin molecules in the skin, an isomeric, water-soluble, renally secretable bilirubin is produced. The effect of phototherapy, i.e., the reduction in the serum bilirubin concentration under phototherapy, may be described as a simple e-function. The evaluation of this regular occurrence provides important information applicable to the phototherapy procedure: it should not be initiated prematurely, the duration of radiation should be as short as possible, the irradiated surface as large as possible, the radiation source should be exploited to a maximum by keeping the distance from the light source short and using lateral reflectors. As supporting measures intestinal lavage, early oral administration of dextrose or oligosaccharide solution and possibly in some cases also human albumin are very useful.