There are limited data on vitamin D insufficiency in healthy children. The aim of this study was to describe the prevalence and determinants of vitamin D insufficiency and its association with bone turnover in adolescent boys (N = 136, mean age 16 years). Sun exposure and physical activity were assessed by questionnaire. Vitamin D stores were assessed by serum 25-hydroxyvitamin D3 (25[OH]D3). Bone turnover was assessed by bone-specific alkaline phosphatase (BAP) and urinary pyridinoline (PYR) to creatinine (Cr) ratio (mmol PYR/micromol Cr). The mean 25(OH)D3 level was low (44 nmol/l; 68% < 50 nmol/l; range, 16-87) and was associated with self-reported sun exposure on winter weekends (r = 0.23, p = 0.01), school holidays (r = 0.22, p = 0.01), and weekdays (r = 0.17, p = 0.05). It was also associated with number of sports (r = 0.34, p < 0.001) and vigorous activity (r = 0.22, p = 0.01) but not television, computer, and video watching (r = -0.04, p = 0.68). In multivariate analysis, number of sports but not total sun exposure remained significantly associated with 25(OH)D3. Furthermore, 25(OH)D3 was significantly associated with BAP in cutpoint analysis (cutpoint 55 nmol/l, p = 0.03) but not continuous analysis (r = -0.12, p = 0.16) and PYR in both forms (r = -0.23, p = 0.01, cutpoint 43 nmol/l, p = 0.01). In conclusion, vitamin D insufficiency is common in healthy adolescent boys in winter in our setting, is primarily derived from sports-related sun exposure, and is associated with bone turnover markers. These data suggest that a 25(OH)D3 level of at least 43-55 nmol/l is required for optimal bone health in children.