Overweight children have reduced exercise capacity and increased aortic stiffness conferring an increased risk of future cardiovascular disease. High intensity exercise training has been shown to improve exercise capacity and reverse aortic stiffness in obese adults. PURPOSE: To describe a novel exercise intervention designed to improve exercise capacity and determine its role in modifying vascular stiffness in obese children. METHODS: Ten to 18 year old children with a BMI ≥ 97th percentile for age are currently being recruited. Using a prospective randomized crossover design, subjects undergo a 12-week supervised high intensity exercise program. During the exercise phase, subjects attend twice-weekly, 75 minute periodized circuit training sessions. A resting echocardiogram and cardiopulmonary treadmill exercise test are performed before and after both the exercise intervention and a 12-week control phase where they receive no intervention. There is an 8-week wash-out period between study phases. Standardized spirometry is used to assess pulmonary mechanics. Physical activity is quantified using tri-axial accelerometry during both study phases. RESULTS: To date 13 subjects have been enrolled; of these, we have complete data for 5 subjects (age 16+3.2 years; ht. 169.4+17.6 cm; wt. 108.1+31.7 kg), 2 have withdrawn prior to starting the protocol, 2 withdrew part way through the protocol, and 4 are in progress. Participants completing the exercise program have attended a minimum of 75% of the required 24 scheduled workouts. Following the exercise intervention, there was a 6% increase in VO2 (L/min) (pre-intervention 3.391+1.023; post-intervention 3.611+1.039) Accelerometry data provided at least three days of physical activity counts for each study phase. All subjects had healthy pulmonary function testing. CONCLUSION: A high intensity exercise training program is feasible in obese children and may improve VO2. Challenges in recruitment and retention in this population of children requires consideration. Funding: 2013-14 British Columbia Children’s Hospital Foundation Telethon Projects Competition