Introduction. Phenylketonuria is a genetic disease that can be diagnosed easily and treated promptly avoiding long-term disabilities. Nevertheless, some children still lack neonatal screening as well as appropriate diagnosis, and they may present serious irreversible neurological damage. Case report. We report 3 cases. Case 1 is a 12 month-old female with motor and developmental delay. Phenylalanine levels were 1 285 μmol/L (normal values 31-75 μmol/L). Her sister (case 2) was a 6 year-old mentally retarded child previously thought to be due to hypoxic-ischemic encephalopathy; her phenylalanine levels were 1 729 μmol/L. Case 3 describes a 10 year-old female with developmental delay, hyperactivity, anxiety, irritability, microcephaly, light-colored hair, and white skin. Phenylalanine levels were 1 170 μmol/L. A low-phenylalanine diet was prescribed for each patient. One month later they were evaluated and showed significantly reduced phenylalanine levels (50%). Currently, they maintain normal values and show important physical and neurological improvement. Conclusions. In cases of late-diagnosed phenylketonuria, a prompt treatment with a strict nutritional management may revert some of the neurological damage developed in these patients.