Phenylketonuria (PKU) is the most frecuent inborn error of metabolism. It is produced by a defect in the phenylalanine hidroxilase (PAH) gen, inducing hyperphenylalaninemia, with irreversible damage in the central nervous system. Early diagnosis and a phenylalanine (Phe) restricted diet prevent mental retardation. In 1988 a newborn screening program for PKU was started; until now we have diagnosed 40 PKU cases. We present 23 PKU children detected through the national screening program and 17 who were born in private clinics. Diagnosis was made in average at 20 ± 12.6 days of age (range 2-60 days) and the Phe levels in serum at begining was 27.14 ± 8.65 mg%. All children were examined to rule out a DHPR cofactor deficiency, at the same time molecular genetics was studied. The follow up included medical, nutritional, biochemical, neurological and psychometric periodic evaluations. Treatment was based on a restricted Phe intake (15-45 mg/wt/day), 1.5 ± 3.0 g/prot/wt/day, 90-130 kcal/wt/day. Phe plasma levels was maintained in the range of 2-10 mg%. Patients had normal growth (between percentils 10 to 90, NCHS tables. Patients are evaluated by Bayley scale until 30 month of age. In later agesthe Stanford-Binet Scale was used. 90% of PKU children remained having normal psychometric evaluation. The remaining 10% were bordeline and all of them have other associate pathologies. We emphasize that early diagnosis and strict follow up permit normal mental and psychomotor development.