Serum threonine concentration was determined during the first month of life in 163 low birthweight infants fed on either human milk, various adapted formulae, or total parenteral nutrition. On the pooled data, a significant positive relationship was found between the serum threonine concentration and threonine intake. However, the increase of the serum threonine level is more marked in the infants with the lowest actual gestational age; with a high threonine intake, the most premature infants have serum threonine levels twice as high (58.1 vs 31.7 μM/dl) as term infants. Therefore, threonine metabolism seems to be impeded in preterm infants. Considering the cord blood concentration of threonine (26.8 $pL 5.1 μM/dl) and the possible hazardous effect of hyperthreoninemia, it is suggested that threonine intake should not exceed 1200 μM (143 mg)/kg bodyweight/day in premature infants and that the amino acid composition of the diet should probably be modified in order to satisfy their protein requirement.