In a cross-sectional study, the primary objective was to assess the plasma concentration of 25-hydroxyvitamin D (25(OH)D) in healthy 9-month-old infants (n=255). The secondary objective was to evaluate nutritional variables and season in relation to 25(OH)D. The concentration of 25(OH)D was 77.2 ± 22.7 nmol/l (mean ± s.d.), ranging from 12 to 151 nmol/l. During the first 9 months, 97% received vitamin D supplementation (10 μg/day) and 89% had sufficient levels of 25(OH)D (50-250 nmol/l). In multiple regression analysis, controlled for body mass index (BMI) and intake of infant formula, a longer period of exclusive breastfeeding (P=0.026) and breastfeeding at 9 months (P=0.001) were both associated with lower levels. Dietary vitamin D intake was 4.4 ± 3.1 μg/day and in multiple regression analysis, controlled for BMI, intake of infant formula and mean energy intake, it was positively associated with 25(OH)D (P=0.001). There was a significant seasonal difference in 25(OH)D, with higher levels during summer-autumn compared with winter-spring (P=0.021) after control for BMI.