Using a novel 3-D ultrasound system, we aimed to determine differences in carotid plaque size and echogenicity in two atherosclerotic groups. Seventy patients admitted with acute myocardial infarction (aMI) and 69 patients known with chronic peripheral arterial disease (cPAD) were included. The cPAD group had larger plaque volumes (median: 70.24 mm³, interquartile range [40.12–135.61] vs. 55.41 mm³ [4.24–84.31], p = 0.004), thicker plaques (2.45 mm [1.85–3.25] vs. 1.99 mm [1.55 – 2.64], p = 0.005) and higher gray-scale medians (GSMs) (mean: 71.75, standard deviation: 21.55 vs. 60.99 [24.09], p = 0.006) than the aMI group. After adjustment for traditional risk factors, the difference persisted for thickness and volume. The difference in GSM persisted after adjustment for volume only. Patients with stable atherosclerotic disease had larger and brighter carotid plaques compared with unstable atherosclerotic patients. 3-D ultrasound may prove useful in identifying thromboembolic risk.