Phenylketonuria (PKU) is due to of a defect in the phenylalanine hydroxylase gene (12q22-24.1) leading to hyperphenylalaninemia. Treatment consists in a low phenylalanine (Phe) diet. Aim: To evaluate the evolution of early diagnosed PKU children, receiving direct breast feeding, and a special formula without Phe, during their first six months of life. Patients and methods: Nineteen PKU children diagnosed in the neonatal period (19.29±13.8 days of age), treated with breast feeding and formula without Phe since diagnosis, were studied. Intake of calories, proteins and dietary Phe were quantified. Blood Phe, nutritional status and psychomotor development were also measured. Results: The diet that these children received during the 6 months period of study, had a mean of 127±19.9 Kcal/kg/day, 1.95±0.3 g protein/kg/day and 35.3±9.5 mg Phe/kg/day. Fifteen children maintained the blood level of Phe under 8 mg/dl, considered an excellent metabolic control. Only 4 cases had intermittently high levels, between 10-12 mg/dl. At 6 months of age, 74% of the children maintained breast feeding as the only source of Phe. Sixty three percent had a normal nutritional status, 5.2% were at nutritional risk and 31.6% were overweight. Eighty one percent had a normal mental development. Conclusions: The use of direct breast feeding allows a good metabolic control and improves growth and development of early diagnosed PKU children (Rev Méd Chile 2003; 131: 1280-87)