The maternal phenylketonuria (MPKU) syndrome is an example of biochemical teratogenesis caused by high phenylalanine concentrations in serum of a pregnant woman (over 360 micromol/L). Active transport through the placenta increases 1.5-fold the phenylalanine level in the child's blood as compared to concentrations recorded in the mother. Increased phenylalanine concentrations may lead to disorders in proliferation of neural cells, neuronal migrations and affect the process of synaptogenesis and myelination. The authors present two children with maternal phenylketonuria with a characteristic clinical picture. Particular attention was drawn to the fact of diagnosed phenylketonuria in mothers following a suspicion and diagnosis of maternal phenylketonuria in children, as well as the occurrence of refractory epilepsy in one of the patients. The mothers' average phenylalanine concentration exceeded the value of 1300 micromol/L, while in children it ranged between 117-160 micromol/L.