Prednisone, a glucocorticoid medication, is widely used for many pediatric disorders. Studies have shown that this drug decreases bone formation, decreasing bone density and bone thickness in children. Prednisone induced osteopenia, or low bone density, can be worsened by the effects of the underlying disease, such as delayed growth and maturation, malnutrition, and increased bone resorption (removal) by inflammatory compounds. The combined effects of decreased bone formation and increased resorption may be particularly detrimental to the growing skeleton.

Subjects will include 15 newly diagnosed NS patients, 60 patients with pre-existing NS, 90 patients with newly diagnosed CD, 45 patients diagnosed within the last two years and 200 healthy controls of similar age, gender and ethnicity. Participants will visit the Children's Hospital of Philadelphia (CHOP) three times over a 12-month period for assessment of bone mineralization and turnover, fracture history, dietary calcium intake, physical activity, growth, body composition, muscle strength and glucocorticoid exposure.