IGF- I is important for somatic growth and development of the human fetus an d neonate. IGF- I also plays an important role in normal vascularization of hum an retina, as it has been suggested that insufficient IGF- I may be a factor in the development of retinopathy of prematurity. The principal regulator of the b ioavailability of IGF- I in the circulation is IGF binding protein 3 (IGFBP- 3 ). The aim of this study was to study factors associated with postnatal serum co ncentrations of IGF- I and of IGFBP- 3 in preterm infants from birth to an age corresponding to 40 wk postmenstruation. We conducted a prospective, longitudin al study in which we measured serum IGF- I and IGFBP- 3 concentrations in 76 p reterm infants from birth (postmenstrual ages 23- 32 wk) until discharge from h ospital around 40 wk. Information regarding nutrition, weight gain, maternal fac tors, and treatment with corticosteroids were collected weekly. Variables found to be associated with postnatal change over time of serum IGF- I and IGFBP- 3 were postmenstrual age (p < 0.001), weight gain (standard deviation score) (p < 0.001), and enteral intake of protein (p < 0.001). Male gender was associated wi th lower IGF- I levels (p < 0.001). The relationship between protein intake and IGF- I (and also between protein intake and IGFBP- 3) was positive, as was th e relationship between weight gain and IGF- I (and between weight gain and IGFB P- 3). These results indicate that the degree of prematurity, low enteral prote in intake, male gender, and slow weight gain are associated with a slower postna tal increase of IGF- I in preterm infants. IGF- I is important for somatic growth and development of the human fetus an d neonate. IGF- I also plays an important role in normal vascularization of hum an retina, as it has been suggested that that IGF- I may be a factor in the development of retinopathy of prematurity. The principal regulator of the b ioavailability of IGF-I in the circulation is IGF binding protein 3 (IGFBP-3). The aim of this study was to study factors associated with postnatal serum levels and of IGFBP-3 in preterm infants from birth to an age corresponding to 40 wk postmenstruation. We conducted a prospective, longitudin al study in which we measured serum IGF- I and IGFBP-3 concentrations in 76 p reterm infants from birth (postmenstrual ages 23-52 wk) until discharge from hospital around 40 wk. Information regarding nutrition, weight gain, maternal fac tors, and treatment with corticosteroids were collected weekly. Variables found to be associated with postnatal change over time of serum IGF-I and IGFBP-3 were postmenstrual age (p <0.001), weight gain (standard deviation score) (p <0.001), and enteral intake of protein The relationship between protein intake and IGF-I (and also between protein intake and IGFBP-3) was positive, as was e relationship between weight gain and IGF-I (and between weight gain and IGFB P- 3). These results indicate that the degree of prematurity, low enteral prote in intake, male gender, and slow weight gain are associated with a slower postna tal increase of IGF-I in preterm infants.