The purpose of this study is to determine whether calcium supplementation improves the bone mineral density of children with SS type sickle cell disease (SCD-SS). Previous studies have shown poor bone mineralization in this group of patients. This 12 month prospective randomized placebo-controlled trial will test whether calcium supplementation will result in significantly improved annual increments in bone mineralization in children with SCD-SS. 56 children, ages 5 to 15 years of age, with SCD-SS and no other conditions known to affect growth or nutritional status, will be enrolled. Enrollment is expected to begin within the next 6 months. Visits will take place at baseline, and at months 6 and 12. Assessments include bone density by dual energy x-ray absoprtiometry, peripheral quantitative computerized tomography and ultrasound. In addition, growth, body composition, puberty status, and dietary intake will be assessed.