Seasonal differences in bone mineral indices have not been studied in newborn infants. In adults, indicators of bone metabolism may show seasonal variations. In postneonatal infants and possibly in adults, vitamin D metabolism shows seasonal variations. We hypothesized that in winter-born infants, the bone mineral content is low and serum osteocalcin is high, related to increased bone turnover and high serum 1,25-dihydroxyvitamin D [1,25(OH)2D]. We studied 246 healthy, term appropriate-for-gestation infants in winter (January through March; 140 children) and summer (July through September; 106 children). The bone mineral content (BMC) of the one-third distal radius was measured before 3 days of age by photon absorptiometry. Significant seasonal differences were found: summer-born infants had significantly lower BMC, higher serum osteocalcin and 1,25(OH)2D, and lower serum total calcium than winter-born infants. Seasonal differences in BMC remained significant after adjusting for race and sex. BMC was not correlated with serum biochemical measures. Thus, summer-born newborn infants have low BMC and high serum osteocalcin and 1,25(OH)2D than winter-born infants; these findings are the opposite of adult findings. We suggest that seasonal effects on fetal bone operate especially in early pregnancy (approximately 6 months before birth) resulting in a "phase effect" and opposite findings from later life.