The aim of this study was to clarify whether atherosclerotic plaque morphology, as defined by quantitative analysis with intravascular ultrasound (IVUS) images, was related to the immunohistochemical findings. Twenty-five coronary lesions in 25 patients who had ultrasound guidance during directional coronary atherectomy (DCA) were enrolled. The lesions retrieved by DCA were analyzed and divided into 3 groups (lesions infiltrated with both macrophages and lymphocytes: group IML; lesions infiltrated with macrophages but not lymphocytes: group IM; and non-infiltrated lesions: group NI). The mean plaque echo level divided by the mean adventitia echo level (MPEL/MAEL) and the heterogeneity of the distribution of plaque echo levels (HDPEL) were calculated. The proportion of patients with acute coronary syndromes was significantly different among the groups: IML (n=14), IM (n=5), and NI (71%, 0% and 17%, respectively; p<0.01). The pre-DCA HDPEL value was highest in group IML and lowest in group NI; however, no significant differences in MPEL/MAEL values were found. The results suggest that plaque morphology, as defined by IVUS images, was related to the immunohistochemical findings. The increase in HDPEL correlated with the presence of immune inflammation. (Circ J 2002; 66: 173 - 177)