This study looks at the effects of taking calcium pills on bone health in young people with juvenile rheumatoid arthritis (JRA). In this 2-year study, children aged 6-18 who have JRA will take either a calcium supplement or a matching placebo (inactive or "dummy" pill) containing no calcium. During the study, researchers and patients will not know if a patient is taking calcium or placebo. We believe that patients who take calcium supplements will have at least a 10 percent greater increase in total body bone mineral density compared to patients who take the placebo.