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Data in humans and nonhuman primates have suggested a possible synergistic effect of vitamin D and calcium (CaD) and estrogen on the cardiovascular disease (CVD) risk factors. Using randomized trial data we explored whether the effect of menopausal hormone therapy (HT) on CVD events is modified by CaD supplementation.A prospective, randomized, double-blind, placebo-controlled trial was implemented among postmenopausal women in the Women's Health Initiative. A total of 27,347 women were randomized to the HT trials (0.625 mg/d of conjugated equine estrogens [CEE] alone for women without a uterus vs placebo; or 0.625 mg of CEE in addition to 2.5 mg of medroxyprogesterone acetate daily [CEE + MPA] for women with a uterus vs placebo). After 1 year, 16,089 women in the HT trial were randomized to the CaD trial and received either 1,000 mg of elemental calcium carbonate and 400 IU of vitamin D3 daily or placebo. The mean (SD) duration of follow-up after CaD randomization was 6.2 (1.3) years for the CEE trial and 4.6 (1.1) years for the CEE + MPA trial. CVD and venous thromboembolism events evaluated in this subgroup analysis included coronary heart disease, stroke, pulmonary embolism, all-cause mortality, plus select secondary endpoints (total myocardial infarction, coronary revascularization, deep venous thrombosis, cardiovascular death, and all CVD events). Time-to-event methods were used and models were fit with a Cox proportional hazards regression model.In the CEE trial, CaD significantly modified the effect of CEE on stroke (P interaction = 0.04). In the CaD-placebo group, CEE's effect on stroke was harmful (hazard ratio [95% confidence interval] = 2.19[1.34-3.58]); however, it was neutral in the CaD-supplement group (hazard ratio [95% confidence interval] = 1.07[0.66-1.73]). We did not observe significant CEE-CaD interactions for coronary heart disease, total CVD events, or any of the remaining endpoints. In the CEE + MPA trial, there was no evidence that the effect of CEE + MPA on any of CVD endpoints was modified by CaD supplementation.CaD did not consistently modify the effect of CEE therapy or CEE + MPA therapy on CVD events. However, the increased risk of stroke due to CEE therapy appears to be mitigated by CaD supplementation. In contrast, CaD supplementation did not influence the risk of stroke due to CEE + MPA.