Background: Rapid postnatal weight gain is associated with obesity and type 2 diabetes in later life. The influence of rapid weight gain on body composition in early infancy is still unknown and the critical periods of weight gain for later disease are debated. Aims: To investigate the effect of birth weight and rapid weight gain on body composition in the first 6 months of life. Study design: The Generation R Study, a population-based prospective cohort study from fetal life onwards. Subjects and outcome measures: We measured body fat and fat distribution by skinfold thickness at the age of 6 weeks and 6 months in 909 Dutch term infants. Analyses were adjusted for current body mass index, sex and maternal socioeconomic status, pre-pregnancy body mass index, height and duration of breastfeeding. Results: Upward postnatal weight percentile change was associated with increased skinfold thickness, percentage body fat at 6 weeks and 6 months and a larger truncal/peripheral fat ratio at 6 months (p<0.01 for all). Birth weight was inversely associated with truncal/peripheral fat ratio (p<0.01) but not with relative body fat at 6 months. Conclusion: During early postnatal rapid weight gain infants do not grow in all body tissues in equal measure. Instead, they acquire relatively large amounts of fat, which is preferentially distributed to the truncal region. Long term observational studies have to assess if such changes in body composition persist into adulthood. (C) 2008 Elsevier Ireland Ltd. All rights reserved.