 Vitamin D insufficiency VDI is a milder form of hypovitaminosis D defined as a 25OHD level between 15 and 30 ngmL regardless of parathyroid hormone PTH status. Experts disagree on whether to treat VDI, as the clinical benefits of therapy are uncertain. Some experts insist the optimal 25OHD level is 30 ngmL. By contrast, both the Food and Nutrition Board and NIH Evidence Report No. 158 state that insufficient evidence exists to declare the optimal serum 25OHD for bone health, despite review of 170 studies. Consequently, the Food and Nutrition Board cannot determine a recommended daily allowance for vitamin D. Confusion over the optimal 25OHD level results, in part, because previous trials failed to recruit subjects based on initial 25OHD levels andor failed to target or achieve 25OHD levels 30 ngmL. Moreover, secondary HPT, the proposed mechanism by which VDI causes bone loss, occurs in only 10 to 33 of people with VDI. As such, people with VDI and normal PTH might not experience clinical benefits from vitamin D therapy. VDI is widespread, affecting 26 to 39 of postmenopausal American women with and without osteoporosis. Therefore, determining the ideal 25OHD level for optimal calcium homeostasis and bone health is of utmost clinical and public health importance. Our overall goal, congruent with Healthy People 2010 objective 2-9, is to evaluate the effect of vitamin D therapy on the risk of osteoporosis in postmenopausal women with VDI, as reflected by changes in CaAb, BMD and muscle fitness. Our second goal is to evaluate whether a high-dose vitamin D regimen, chosen to achieve and maintain a 25OHD level 30 ngmL, is superior in its effects on study outcomes compared to a low-dose vitamin D regimen that can permit continued VDI. 