The calcium/vitamin D supplementation subtrial tests whether calcium and vitamin D supplements reduce the risk of hip and other fractures and colorectal cancer in postmenopausal women. Women in the hormone replacement therapy and the dietary modification trials are encouraged to join the calcium/vitamin D supplementation study. Approximately 45,000 postmenopausal women are randomized into one of two study groups. One group was randomly assigned to receive 1,000 mg of elemental calcium (as calcium carbonate) and 400 International Units of vitamin D₃daily. The second group received a matching placebo.