Children with chronic diseases are at increased risk of sub-optimal bone mineral acquisition and osteoporosis, especially those who have a growth and pubertal delay, reduced physical activity, inadequate nutrition, malabsorption or take medications which may influence bone development. Weight-bearing physical activity has a beneficial effect on bone development of healthy children but little is known in children with chronic diseases. Preliminary results of our cross-sectional study in children with juvenile idiopathic arthritis (JIA) suggest that hip bone mineral density is positively related with physical fitness and muscle strength and is reduced at the more affected side. We have initiated two randomized controlled trials to determine the effects of a moderate impact exercise training program on bone mineral density of children with JIA and type 1 diabetes mellitus.