To determine the effects of length of gestation and sex on infant body composition, air displacement plethysmography was performed in forty-six full-term neonates at 3 d of life and during the week prior to hospital discharge in 180 preterm neonates. Fat mass, as a percentage of body weight, was higher in preterm than in term infants (13.4 (SD 4.2) v. 10.1 (SD 3.7) %, respectively; P= 0.001). The absolute amount of fat mass did not differ between preterm and full-term newborns (323 (SD 126) v. 335 (SD 138) g; P= 0.58), whereas lean body mass was lower in preterm than in term infants (2055 (SD 280) v. 2937 (SD 259) g, respectively; P<0.001). Among full-term infants, fat mass was higher in females than in males (11.1 (SD 3.7) v. 9.0 (SD 3.3) %, respectively; P= 0.047), whereas we did not observe any sex difference in preterm infants (13.5 (SD 4.1) v. 13.4 (SD 4.3) %; P= 0.89). Our data suggest that by the time they are discharged from hospital: (1) preterm infants have a higher percentage of body fat than term neonates and (2) this is presumably due to a lesser accretion in lean body mass in the first few weeks of extra-uterine life, particularly in boys.