Phenylketonuria is an inherited metabolic disorder caused by insufficient activity of the enzyme phenylalanine hydroxylase and the consequent accumulation of harmful amino acids phenylalanine and its metabolites. If the disorder is not treated properly, it can cause severe brain damage that can manifest in psychomotor retardation, epilepsy, small head circumference, condition similar to autism. If the disorder is detected in time and properly treated, the baby is completely healthy. Treatment consists of lifelong diet without phenylalanine. Galactosemia is the result of hereditary lack of enzymes that convert galactose to glucose. Feeding such children with milk can cause severe damage to the liver, kidneys and brain, and can negatively affects vision. But if the disease in the first days of life is detected and treated with a suitable diet, the child remains healthy. The treatment consists of strict avoidance of galactose, lactose component, which is found in breast milk, cow's milk and other foodstuffs. Newborn screening is a systematic search of the entire population of newborns in certain region or the entire country on those diseases that are available in the treatment, which can be early clinical recognized. To each newborn in Croatia before leaving the maternity ward is taken a drop of blood on a piece of paper, sent to the laboratory and in the case that the result is positive children are invited to the clinic for metabolic deseases of Department of Pediatrics, University Hospital Center Zagreb.