PURPOSE: The most recent large-scale studies evaluating the effects of calcium supplementation for prevention of osteoporosis-related fractures in postmenopausal women are reviewed. SUMMARY: Osteoporosis is a very common disease associated with significant morbidity and mortality. For many years, use of a calcium supplement (preferably in combination with vitamin D to optimize calcium absorption) has been recommended for postmenopausal women to decrease fracture risk. However, five large-scale, randomized, controlled trials have called into question the benefits of calcium in reducing fracture risk, and four of the studies indicated that calcium users may be at increased risk for renal stones and gastrointestinal problems. However, all five studies had one or more important limitations, including possible selection bias and study participants' relatively high baseline calcium intake and generally low adherence to treatment regimens. Moreover, in some of the studies, vitamin D was not included in the treatment protocol or was not used at levels sufficient to optimize calcium absorption. In three of the five trials, subgroup analysis of the most treatment-adherent participants indicated significant reductions in osteoporotic fracture risk with calcium supplement use. CONCLUSION: Results of recent clinical trials indicate that calcium supplementation does not significantly reduce fracture risk in postmenopausal women. However, evidence from the same studies suggests that beneficial effects on fracture risk may be seen in women who are adherent to therapy. Postmenopausal women should continue calcium supplementation to reduce osteoporosis risk.