Serum aminoterminal propeptide of type III procollagen (PIIINP) concentration are markers of connective tissue growth and/or degradation. Severe vitamin D (D) deficiency in pregnancy has been associated with decreased intrauterine growth. Potential effects of seasonal D variations on biochemical markers of fetal growth are unknown. We reported significantly low winter newborn serum 25-hydroxyvitamin D (25-OHD) and total bone mineral content (by DXA) vs. summer-born Korean neonates, suggestive of low fetal D status affecting bone mineralization in winter (Pediatr Res 37:314A, 1995). We hypothesized that serum PIIINP would be low in winter vs. summer-born Korean neonates, reflecting reduced collagen production in D deficiency. Cord serum PIIINP was measured by a radioimmunoassay kit (Orion Diagnostica, Finland; adult range 1.7-4.2μg/L). Twenty summer-born (mean±SD; 38.2±0.7wks; 3287±281g) and 19 winter-born Korean neonates(38.3±0.6wks; 3381±163g) were studied(19 males & 20 females); all infants were term appropriate for gestation(AGA). Winter-born AGA neonates have lower serum PIIINP than summer-born (75.2±16.1 vs. 96.6±34.7 μg/L, p=0.0174); seasonal difference remained significant after adjusting for birth weight (p=0.0256). Serum 25-OHD was lower in winter than summer-born infants (5.1±3 vs. 10.4±4ng/mL, p<0.001). Serum PIIINP positively correlated with serum 25-OHD (r=0.364, p=0.0266). Serum PIIINP was not different by sex (males, 86.7±34.2 vs. females, 85.6±23.1 μg/L, p=0.9). Thus, serum PIIINP concentrations were lower in winter than summer-born AGA neonates, and directly correlated with low vitamin D status. We suggest that fetal collagen production rate is different by season of birth, possibly reflecting reduced fetal growth in winter. Season of birth should be considered a factor influencing type III collagen metabolism in newborn infants.