This study looks at the effects of calcium supplementation on bone density in women in their third decade of life. We placed women aged 19-27 who take in low amounts of calcium in their diets in one of two groups. We will give women in one group a placebo (inactive pill) and women in the other group 1500 milligrams of calcium per day (as calcium carbonate). We will monitor the results by looking at the change in bone mineral density measured at the hip, total body, forearm, and spine. Treatment will last 3 years.