The purpose of this study is to compare healthy children to children who have a chronic illness called Juvenile Idiopathic Arthritis (JIA). JIA is a childhood disease that causes swollen joints that are often stiff and painful. JIA affects about 1 in 1,000 children age 16 and younger.

Children with chronic illnesses are at risk for bone fragility due to inflammation, glucocorticoid therapy, physical activity limitation, malnutrition, and pubertal delay. The impact of low bone mass during childhood may be immediate, resulting in childhood fractures, or delayed, due to suboptimal peak bone mass attainment.

Inclusion Criteria:

• for JIA patients: subjects age 5-21 Drawn from Dr. Burnham's prospective cohort study of bone health in 101 children with arthritis.
• for Control patients: subjects age 5-21 Controls will be a 50% male/female

Exclusion Criteria:

• for JIA patients: Subjects with JIA will be excluded if they have conditions or drug exposure unrelated to JIA and known to impact growth or bone health.
• for Control patients: Chronic disease or syndrome known to affect growth or bone health, prematurity (<37 weeks gestation), or use of any medication known to affect growth.

200 control patients and 30 study patients