There is longstanding concern that calcium supplements might increase cardiovascular risk in patients with renal impairment. The Auckland Calcium Study suggested that the same problem occurs in older people taking these supplements for prevention of osteoporosis. Our subsequent meta-analyses, (which followed protocols finalized before the data was available) confirmed that calcium supplements, with or without vitamin D, adversely affected risk of myocardial infarction and, possibly, stroke. Several groups have re-visited these data, consistently finding an adverse effect of calcium on myocardial infarction, not always statistically significant because some meta-analyses have been under-powered. Whether or not an adverse effect of calcium plus vitamin D on myocardial infarction is found depends on whether two specific groups of subjects are included-those in the Women's Health Initiative who were already taking calcium at the time of randomization, and subjects from an open, cluster-randomized study in which baseline cardiovascular risk was different between groups. Vitamin D alone does not affect vascular risk, so it is unlikely that differences between calcium alone and calcium plus vitamin D are real, and they are more likely to result from the inclusion of studies at high risk of bias. The mechanisms of the adverse cardiovascular effects are uncertain but may be mediated by the increase in serum calcium following supplement ingestion, and the effects of this on vascular function and coagulation. Available evidence suggests the risks of calcium supplements outweigh any small benefits on fracture incidence, so the case for their use is weak.