Importance:The role of coronary artery calcium (CAC) testing for guiding preventive strategies among women at low cardiovascular disease (CVD) risk based on the American College of Cardiology and American Heart Association CVD prevention guidelines is unclear. Objective:To assess the potential utility of CAC testing for CVD risk estimation and stratification among low-risk women. Design, Setting, and Participants:Women with 10-year atherosclerotic CVD (ASCVD) risk lower than 7.5% from 5 large population-based cohorts: the Dallas Heart Study (United States), the Framingham Heart Study (United States), the Heinz Nixdorf Recall study (Germany), the Multi-Ethnic Study of Atherosclerosis (United States), and the Rotterdam Study (the Netherlands). The 5 cohorts were selected based on the availability of CAC data in a sizable group of low-risk women from the general population together with the long detailed follow-up data. Across the cohorts, events were assessed from the date of CAC scan (performed from 1998 through 2006) until January 1, 2012; January 1, 2014; or March 6, 2015. Fixed-effects meta-analysis was conducted to combine the results of the 5 studies. Exposures:CAC score by computed tomography. Main Outcomes and Measures:Main outcome was incident ASCVD, including nonfatal myocardial infarction, coronary heart disease (CHD) death, and stroke. Association of CAC with ASCVD was examined using Cox proportional hazards models. To assess whether CAC was associated with improved ASCVD risk predictions beyond the traditional risk factors, the C statistic and the continuous net reclassification improvement (cNRI) index were calculated. Results:Among 6739 women with low ASCVD risk from the 5 studies, mean age ranged from 44 to 63 years and CAC was present in 36.1%. Across the cohorts, median follow-up ranged from 7.0 to 11.6 years. A total of 165 ASCVD events occurred (64 nonfatal myocardial infarctions, 29 CHD deaths, and 72 strokes), with the ASCVD incidence rates ranging from 1.5 to 6.0 per 1000 person-years. Compared with the absence of CAC (CAC = 0), presence of CAC (CAC >0) was associated with an increased risk of ASCVD (incidence rates per 1000 person-years, 1.41 for CAC absence vs 4.33 for CAC presence; difference, 2.92 [95% CI, 2.02-3.83]; multivariable-adjusted hazard ratio, 2.04 [95% CI, 1.44-2.90]). The addition of CAC to traditional risk factors improved the C statistic from 0.73 (95% CI, 0.69-0.77) to 0.77 (95% CI, 0.74-0.81) and provided a cNRI of 0.20 (95% CI, 0.09-0.31) for ASCVD prediction. Conclusions and Relevance:Among women at low ASCVD risk, CAC was present in approximately one-third and was associated with an increased risk of ASCVD and modest improvement in prognostic accuracy compared with traditional risk factors. Further research is needed to assess the clinical utility and cost-effectiveness of this additional accuracy.