It was originally suggested (by the applicants) that dietary calcium is an important determinant of bone mass in young adults. "Who may need calcium, how much, and for how long?" are still unanswered questions. To address these questions, 311 Caucasian females, initially in pubertal stage 2 (average age, 11 years) and with calcium intake less than 1200 mg/day (current RDA for calcium) were recruited for a clinical trial and randomized into groups of approximately 150. One group received placebo while the other received 1 gram of calcium from calcium citrate-malate. Calcium efficacy has been assessed by food record, anthropometry, blood and urine chemistry, metabolic balance studies, and bone mass measurements every 6 to 12 months. The project was originally approved as a 4 year longitudinal study and there is 6 months remaining. The goal of the research proposed in this competing renewal application is to continue this ongoing double-blind placebo-controlled clinical trial with calcium for an additional 3 years, which will allow for continuous measurements of biological variables of interest in this group of already committed young females (from age approx. 11-18 years) from childhood to adulthood. It is suggested that continuation of the project will provide: 1) definitive data about the long-term effects of calcium on bone mass of growing females through phases of bone modelling (puberty) and skeletal consolidation (late adolescence, young adulthood); 2) a strategy for the primary prevention of osteoporosis based on molecular epidemiology; 3) "bone density charts" for growing females; 4) data about chronic effects of urinary calcium and sodium on bone mass acquisition; and 5) long-term data about the effect of calcium on blood pressure in young individuals when traits of hypertension are being established. This 7-year double-blind placebo-controlled clinical trial (in conjunction with previous epidemiologic studies) is intended to provide unique information about the prophylactic role of calcium in osteoporosis by maximizing bone mass during critical years of skeletal development, when calcium requirements are highest and calcium intake declines. The applicant suggests that to answer these new and compelling scientific questions, an independent study would be substantially more costly than the one proposed in this competitive continuation.