OBJECTIVE: Experimental and epidemiologic studies suggest that calcium and vitamin D may reduce the risk of developing diabetes. We examined the effect of calcium plus vitamin D supplementation on the incidence of drug-treated diabetes in postmenopausal women. RESEARCH DESIGN AND METHODS: The Women's Health Initiative Calcium/Vitamin D Trial randomly assigned postmenopausal women to receive 1,000 mg elemental calcium plus 400 IU of vitamin D3 daily, or placebo, in a double-blind fashion. Among 33,951 participants without self-reported diabetes at baseline, we ascertained by treatment assignment new diagnoses of diabetes treated with oral hypoglycemic agents or insulin. Effects of the intervention on fasting measurements of glucose, insulin, and insulin resistance were examined among a subset of participants. RESULTS: Over a median follow-up time of 7 years, 2,291 women were newly diagnosed with diabetes. The hazard ratio for incident diabetes associated with calcium/vitamin D treatment was 1.01 (95% CI 0.94-1.10) based on intention to treat. This null result was robust in subgroup analyses, efficacy analyses accounting for nonadherence, and analyses examining change in laboratory measurements. CONCLUSIONS: Calcium plus vitamin D3 supplementation did not reduce the risk of developing diabetes over 7 years of follow-up in this randomized placebo-controlled trial. Higher doses of vitamin D may be required to affect diabetes risk, and/or associations of calcium and vitamin D intake with improved glucose metabolism observed in nonrandomized studies may be the result of confounding or of other components of foods containing these nutrients.