The objective of this study was to determine whether vitamin D supplementation of breast-fed infants during the first year of life is associated with greater bone mineral content and/or areal bone mineral density (aBMD) in later childhood. The design was a retrospective cohort study. One hundred and six healthy prepubertal Caucasian girls (median age, 8 yr; range, 7-9 yr) were classified as vitamin D supplemented or unsupplemented during the first year of life on the basis of a questionnaire sent to participating families and their pediatricians. Bone area (square centimeters) and bone mineral content (grams) were determined by dual energy x-ray absorptiometry at six skeletal sites. Vitamin D receptor (VDR) 3'-gene polymorphisms (BsmI) were also determined. The supplemented (n = 91) and unsupplemented (n = 15) groups were similar in terms of season of birth, growth in the first year of life, age, anthropometric parameters, and calcium intake at time of dual energy x-ray absorptiometry. The supplemented group had higher aBMD at the level of radial metaphysis (mean +/- SEM, 0.301+/-0.003 vs. 0.283+/-0.008; P = 0.03), femoral neck (0.638+/-0.007 vs. 0.584+/-0.021; P = 0.01), and femoral trochanter (0.508+/-0.006 vs. 0.474+/-0.016; P = 0.04). At the lumbar spine level aBMD values were similar (0.626+/-0.006 vs. 0.598+/-0.019; P = 0.1). In a multiple regression model taking into account the effects of vitamin D supplementation, height, and VDR genotype on aBMD (dependent variable), femoral neck aBMD remained higher by 0.045 g/cm2 in the supplemented group (P = 0.02). Vitamin D supplementation in infancy was found to be associated with increased aBMD at specific skeletal sites later in childhood in prepubertal Caucasian girls.