The effect of low or borderline vitamin D status on bone mineralization of exclusively breast milk-fed infants has not been studied. The low vitamin D status of Korean breast milk-fed infants may theoretically have adverse effects on bone mineralization. Assuming that bone mineral content (BMC) relates in part to vitamin D status, we hypothesized that serum 25-hydroxyvitamin D (25-OHD) concentration and BMC would be low, and serum osteocalcin concentration high, reflecting active bone turnover, in breast milk- versus formula-fed infants born in the winter. Eighteen breast milk- and 17 formula-fed infants were recruited at ages 2 to 5 months. The BMC of the lumbar1-4 spine region was measured by using dual energy x-ray absorptiometry. The BMC and serum osteocalcin levels were similar for both groups. The serum 25-OHD level was significantly lower in breast milk- than formula-fed infants; 44% of the breast milk group versus 6% of the formula group had serum 25-OHD levels less than 28 nmol/L (11 ng/ml), the lower limit of normal. The BMC did not correlate with the serum 25-OHD level. Thus BMC and serum osteocalcin levels in 2- to 5-month-old infants were not different by type of feeding, despite low vitamin D status in breast milk-fed infants. We speculate that adequate mineral absorption occurs during this period from a predominantly (vitamin D independent) passive transport mechanism. (J Pediatr 1998;132:641-5).