OBJECTIVE: This study aims to test the added value of calcium and vitamin D (CaD) in fracture prevention among women taking postmenopausal hormone therapy (HT). METHODS: This is a prospective, partial-factorial, randomized, controlled, double-blind trial among Women's Health Initiative postmenopausal participants aged 50 to 79 years at 40 centers in the United States with a mean follow-up of 7.2 years. A total of 27,347 women were randomized to HT (0.625 mg of conjugated estrogens alone, or 0.625 mg of conjugated equine estrogens plus 2.5 mg of medroxyprogesterone acetate daily), and 36,282 women were randomized to 1,000 mg of elemental calcium (carbonate) plus 400 IU of vitamin D3 daily, each compared with placebo. A total of 16,089 women participated in both arms. The predefined outcomes were adjudicated hip fractures and measured bone mineral density. RESULTS: Interaction between HT and CaD on hip fracture (P interaction = 0.01) was shown. The effect of CaD was stronger among women assigned to HT (hazard ratio [HR], 0.59; 95% CI, 0.38-0.93) than among women assigned to placebo (HR, 1.20; 95% CI, 0.85-1.69). The effect of HT on hip fracture was stronger among women assigned to active CaD (HR, 0.43; 95% CI, 0.28-0.66) than among women assigned to placebo (HR, 0.87; 95% CI, 0.60-1.26). CaD supplementation enhanced the antifracture effect of HT at all levels of personal calcium intake. There was no interaction between HT and CaD on change in hip or spine bone mineral density. CONCLUSIONS: Postmenopausal women at normal risk for hip fracture who are on CaD supplementation experience significantly reduced incident hip fractures beyond HT alone at all levels of personal baseline total calcium intake. © 2013 by The North American Menopause Society.