We evaluated the incidence and predictors of edge dissection of the calcified culprit plaque in patients with acute coronary syndrome (ACS) or stable angina (SA). Calcified plaque is not rare in patients with ACS, and compliance mismatch may create edge dissection of the calcified plaque to trigger ACS. However, little data are available on calcium edge dissection in relation to ACS. Pre-intervention intravascular ultrasound data were analyzed in 143 patients with ACS (n = 53) or SA (n = 90). Edge dissection of the calcified plaque was found in 14 patients (9.8%). Patients were divided into two groups based on calcium edge dissection: group I (edge dissection, n = 14) and group II (no edge dissection, n = 129). Clinical and angiographic characteristics were largely similar between the two groups; however, ACS was more common in group I than in group II (64.3 vs. 34.1%, respectively, p = 0.039). Intravascular ultrasound variables did not differ between the two groups except thrombus and reference measurements, with thrombus more frequently observed in group I than in group II (35.7 vs. 8.5%, respectively, p = 0.010). Likewise, proximal and distal reference measurements were larger in group I than in group II. Multivariate analysis showed that ACS was the only independent predictor of calcium edge dissection (odds ratio 3.5, 95% confidence interval 1.1-11.0, p = 0.034). Edge dissection of the calcified plaque was present and more common in ACS patients than in SA patients. Calcium edge dissection may play a role in the pathogenesis of ACS.