BACKGROUND: Diabetes mellitus is an established cardiovascular risk factor. We assessed the impact of diabetes mellitus on quantitative plaque and long-term outcomes in patients with and without diabetes mellitus in the Scottish COmputed Tomography of the HEART (SCOT-HEART) trial. METHODS: Coronary artery calcium (CAC) was assessed on non-contrast computed tomography (CT). Coronary stenoses, visually assessed adverse plaque characteristics and quantitative plaque burdens (calcified, non-calcified, low attenuation and total, Autoplaque 2.5) were assessed on coronary CT angiography. Multivariable and survival analyses were performed. RESULTS: Images of 1769 patients were assessed (56 % male, 58 ± 9 years). Diabetes mellitus was present in 196 (11 %) patients. Patients with diabetes mellitus had higher 10-year cardiovascular risk score (29 [interquartile range 21, 40] versus 15 [9, 21] %, p <
0.001) and CAC score (71 [1, 447] versus 17 [0, 209] Agatston units, p <
0.001), but were not more likely to have obstructive disease or visually assessed adverse plaque characteristics. Patients with diabetes mellitus had higher quantitatively assessed calcified, non-calcified, low attenuation and total plaque burdens. After adjustment for age and sex, diabetes mellitus was an independent predictor of calcified plaque burden (p = 0.009), but not the other plaque types. During 8.7 [IQR 8, 9.6] years follow-up, diabetes mellitus was associated with an increased risk of fatal or non-fatal myocardial infarction, adjusted for age and sex (hazard ratio 1.85, 95 % confidence interval 1.09 to 3.17, p = 0.024). CONCLUSION: Diabetes mellitus was an independent predictor of quantitatively assessed plaque burden, particularly calcified plaque, and was associated with an increased risk of myocardial infarction.