Six cases of phenylketonuria (PKU) are reported. The patients belong to three different families — three siblings. Genealogic, clinic and laboratorial studies have been done, including serum phenylalanin. The important aim was to show the normality of the histopathology performed through cerebral biopsy in four cases. Histochemical study has not been done. The normal cerebral biopsy did not show correlation with mental level, electroencephalographic alterations or with cerebral atrophies which were found in two cases by air encephalography. All cases have been treated with special diet (Lofenalac) and some food with low phenylalanin level. After and before the diet phenylalanin determinations have been done, nevertheless, it is very soon indeed to conclude anything by now.