The effect of calcium carbonate supplementation on bone growth and mineral accretion was studied in 143 boys aged 16-18 yr, randomized to 1000 mg Ca/d or a matching placebo for 13 months. Anthropometry and dual-energy x-ray absorptiometry of the whole body, lumbar spine, hip, and forearm were performed before, during, and after the intervention. The intervention resulted in greater bone mineral content (BMC) of the whole body (+1.3%, P = 0.02), lumbar spine (+2.5%, P = 0.004), and hip (total +2.3%, P = 0.01; neck +2.4%, P = 0.02; intertrochanter +2.7%, P = 0.01). This was associated with greater height (+0.4%, P = 0.0004, equivalent to 7 mm), lean mass (+1.3%, P = 0.02), and lumbar spine bone area (+1.5%, P = 0.003). The increases in BMC diminished after size adjustment, suggesting that the intervention effect was mediated through an effect on growth. The BMC response at the intertrochanter was greater in subjects with high physical activity (+4.4%, P = 0.05). There were no other significant interactions with physical activity, plasma testosterone, calcium intake, or tablet compliance. We conclude that calcium carbonate supplementation of adolescent boys increased skeletal growth, resulting in greater stature and bone mineral acquisition. Follow-up studies will determine whether this reflects a change in the tempo of growth or an effect on skeletal size that persists into adulthood.