Context: Vitamin D is essential for bone health in adolescence, when there is rapid bone mineral content accrual. Because cutaneous sun exposure provides Vitamin D, there is no recommended oral intake for UK adolescents. Objective: Our objective was to assess seasonal Vitamin D status and its contributors in white Caucasian adolescents and examine bone health in those found deficient. Design: Prospective cohort study was undertaken. Setting: Six schools in Greater Manchester, UK, were included. Participants: Participants were 131 adolescents between 12 and 15 years of age. Intervention(s): Seasonal assessment of circulating 25-hydroxyvitamin D (25OHD), personal sun exposure, and dietary Vitamin D. Adolescents deficient (25OHD <10 ng/ml/25 nmol/liter) in at least one season underwent dual-energy X-ray absorptiometry (lumbar spine, femoral neck), with bone mineral apparent density correction for size, and peripheral quantitative computed tomography (distal radius) for volumetric bone mineral density (BMD). Main Outcome Measure: Serum 25OHD and BMD measurements. Results: Mean 25OHD was highest in September: 24.1 (SD, 6.9) ng/ml and lowest in January: 15.5 (5.9) ng/ml. Over the year, 16% were deficient in ≥ one season and 79% insufficient (25OHD <20 ng/ml/50 nmol/liter) including 28% in September. Dietary Vitamin D was low year-round, whereas personal sun exposure was seasonal and predominantly across the school week. Holidays accounted for 17% variation in peak 25OHD (P < .001). Nineteen adolescents underwent bone assessment, which showed low femoral neck bone mineral apparent density vs matched reference data (P = .0002), three with Z less than or equal to -2.0 distal radius trabecular volumetric BMD. Conclusions: Sun exposure levels failed to provide adequate Vitamin D, with approximately onequarter of adolescents insufficient even at summer peak. Seasonal Vitamin D deficiency was prevalent and those affected had low BMD. Recommendations on Vitamin D acquisition are indicated in this age-group.