Technical Abstract: To evaluate the effects on serum 25(OH)D and bone mineralization of supplementation of breast-fed Hispanic and non-Hispanic Caucasian infants with vitamin D in infants in Houston, Texas. We measured cord serum 25(OH)D levels, bone mineral content (BMC), bone mineral density (BMD) and their changes over 3 months of life with 400 IU/day of vitamin D3 supplementation. Cord serum 25(OH)D was significantly lower in Hispanic than non-Hispanic Caucasian infants (16.4 +/- 6.5 ng/mL, n = 27, vs 22.3 +/- 9.4 n = 22, p = 0.013). Among 38 infants who completed a 3 month vitamin D supplementation intervention, provision of 400 IU/day of vitamin D increased final 25(OH)D to a higher level in non-Hispanic Caucasian compared to Hispanic infants. There was no significant relationship between cord serum 25(OH)D and BMC or BMD in the first week of life (n = 49) or after 3 months of vitamin D supplementation. Low cord 25(OH)D levels are seen in Hispanic infants, but their functional significance is uncertain related to bone health in a southern US setting. Daily vitamin D intake of 400 IU during the first months of life appears adequate to increase serum 25(OH)D and support BMC increases despite low initial 25(OH)D levels in some infants.