OBJECTIVE To assess lifetime cardiovascular disease (CVD) risk by coronary artery calcium (CAC) score in individuals with diabetes from the Multi-Ethnic Study of Atherosclerosis (MESA) and compare risk with that in individuals without diabetes. RESEARCH DESIGN AND METHODS We developed a microsimulation model with well, diabetes, post-CVD, and death health states using multivariable time-dependent Cox regression with age as time scale. We initially used 10-year follow-up data of 6,769 MESA participants, including coronary heart disease (CHD) (n = 272), heart failure (n = 201), stroke (n = 186), and competing death (n = 619) and assessed predictive validity at 15 years. We externally validated the model in matched National Health and Nutrition Examination Survey (NHANES) participants. Subsequently, we predicted CVD risk until age 100 years by diabetes, 10-year pooled cohort equations risk, and CAC score category (0, 1–100, or 100+). RESULTS The model showed good calibration and discriminative performance at 15 years, with discrimination indices 0.71–0.78 across outcomes. In the NHANES cohort, predicted 15-year mortality risk corresponded well with Kaplan-Meier risk, especially for those with diabetes: 29.6% (95% CI 24.9–34.8) vs. 32.4% (95% CI 27.2–37.2), respectively. Diabetes increased lifetime CVD risk, similar to shifting one CAC category upward (from 0 to 1–100 or from 1–100 to 100+). Patients with diabetes and CAC score of 0 had a lifetime CVD risk that overlapped with that of individuals without diabetes who were at low 10-year pooled cohort equations risk (<7.5%). CONCLUSIONS Patients with diabetes carry a spectrum of CVD risk. CAC scoring may improve decisions for preventive interventions for patients with diabetes by better delineating lifetime CVD risk.