To obtain more information about the correct protein intake in low birth weight children, serum amino acid concentrations were determined. In 59 formula-fed, low birth weight children, serum amino acid concentrations were determined at 3, 24 and 72 h after birth and at the age of 1, 2, 3 and 4 weeks. Mean serum concentrations of all amino acids decreased during the first 24 h of life, except of aspartate and tyrosine. This decrease was followed by an increase during the first weeks of life and a variable pattern afterwards. A hypertyrosinemia (serum-tyrosine concentration > 250 μmol/l) was not found in any case. Taurine concentrations decreased gradually, as was expected because of the low taurine concentration in cow’s milk. Mean concentrations of alanine, glycine, proline and valine were higher in small for date children. Concluding, parenteral supplementation of amino acids is advisable during the first days of life to low birth weight childr