Prior intravascular ultrasound (IVUS) studies have demonstrated that a positive remodeling pattern of a culprit lesion is observed more frequently in acute coronary syndrome (ACS) than stable angina (SA). However, the relationship between the plaque morphology detected by IVUS and the histological type of atherosclerotic plaque has not been well defined. This is a prospective study on 37 consecutive patients who underwent directional coronary atherectomy. The 37 patients were divided into 2 groups; 21 patients with SA and 16 with ACS. Vessel and plaque cross sectional area were measured at the culprit lesion and the remodeling index (RI) was calculated by IVUS. The plaque tissue was assessed for the presence of inflammatory cells and lipids, and the presence of each was scored as 0 (absent), 1 (sparse), 2 (dense), or 3 (predominant). The RI of the patients with ACS was higher than that of SA. Inflammatory cells were present to a greater extent in patients with ACS. Inflammatory cells and lipids were significantly correlated with the RI (Inflammatory cell score grade ≥ 2 patients; 1.14 ± 0.13 versus grade 0 patients; 0.87 ± 0.24, and grade 1 patients; 0.93 ± 0.17, P < 0.01 and lipid score grade ≥ 2 patients; 1.13 ± 0.17 versus grade 0 patients; 0.85 ± 0.18, P < 0.001 and grade 1 patients; 0.95 ± 0.19, P < 0.05). The results clearly indicate that the evaluation of vessel morphology by vascular imaging is an important indicator of plaque instability.