Fenilketonurija je najčešća urođena greška u metabolizmu aminokiselina. Nastaje zbog nemogućnosti pretvorbe esencijalne aminokiseline fenilalanina u tirozin, najčešće zbog nedovoljne produkcije ili potpunog nedostatka enzima fenilalaninhidroksilaze uslijed mutacije njenog gena. U većini razvijenih zemalja pa tako i u Republici Hrvatskoj otkriva se obaveznim novorođenačkim probirom. Liječenje se provodi strogo reguliranom doživotnom niskoproteinskom dijetoterapijom. U radu je prikazan slučaj jednogodišnje djevojčice s fenilketonurijom koja je dobro regulirana. Phenylketonuria is the most common inborn error of amino acid metabolism. It is caused by the impossibility of converting the essential amino acid phenylalanine into tyrosine, most often due to insufficient production or a complete lack of the enzyme phenylalanine hydroxylase due to a mutation in its gene. In most developed countries, including Croatia, it is detected through mandatory newborn screening. Treatment is carried out with strictly regulated lifelong low-protein diet therapy. The case of a one-year-old girl with phenylketonuria, which is well-regulated, is presented.