Phenylketonuria (Online Mendelian Inheritance in Man 261600) is the most common genetic autosomal recessive disease affecting metabolism. This diet-dependent condition is found in Poland in 1:8,000 live births. Its prevalence in the world (in screened populations) is estimated at 1:12,000, and every 55th individual is a carrier of the defective phenylalanine hydroxylase gene. Phenylketonuria involves the complete absence or partial activity deficit of the phenylalanine hydroxylase enzyme. The metabolic block results in the accumulation of excessive amounts of phenylalanine and its metabolites in body fluids, which leads to central nervous system injury. Adjusting nutrition to the metabolic efficiency of phenylketonuria patients determines proper psychomotor development. A low-phenylalanine diet is the referential treatment method of the classic form of phenylketonuria. Adult women with phenylketonuria in the preconception period and during pregnancy are a particular group of patients. They require a very restrictive low-phenylalanine diet as abnormal blood concentrations of phenylalanine lead to maternal phenylketonuria syndrome, i.e. foetal injury, in pregnant woman with phenylketonuria and poor metabolic control. It is therefore important that adult women with hyperphenylalaninaemia remain under close specialist care (doctors and dieticians), and that gynaecologists, obstetricians, neonatologists, paediatricians and primary care physicians are familiar with maternal phenylketonuria syndrome.