BACKGROUND AND OBJECTIVES: A reduction of the coronary flow reserve (CFR) following successful percutaneous coronary intervention (PCI) is related to microvascular impairment. Embolization of atherosclerotic debris during PCI is a possible explanation for the finding of abnormal coronary Doppler flow following PCI. SUBJECTS AND METHODS: The CFR and intravascular ultrasound (IVUS), both before and after PCI, were recorded in 69 lesions of 69 patients with coronary artery disease. An abnormal CFR was defined as one with no change or a decrease after successful PCI. RESULTS: The patients were divided into abnormal (n=17) and normal CFR (n=52) groups. After stenting, the hyperemic flow velocity was significantly lower in the abnormal CFR group (39.3+/-12.6 vs. 48.9+/-15.4 cm/s, p=0.022). 94 and 29% of the abnormal group had soft plaques and lipid cores, respectively, compared with 62 and 2% in the normal CFR group (soft plaque: p=0.029, lipid core: p=0.002). The abnormal CFR group had smaller post-procedural vessels (15.1+/-4.2 vs. 18.2+/-4.9 mm2, p=0.039) and plaque areas (6.8+/-2.7 vs. 9.9+/-3.8 mm2, p=0.006). Furthermore, the abnormal CFR group showed less vessel expansion (1.7+/-5.5 vs. 5.0+/-3.9 mm2, p=0.018) and greater plaque loss (4.1+/-5.3 vs. 0.7+/-3.4 mm2, p=0.009). The abnormal CFR group had an increased CK-MB following PCI (4 patients, 23.5% vs. 2 patients, 3.8%, p=0.029). In a multivariable analysis, the only predictor of an abnormal CFR was the presence of a lipid core within the plaque. CONCLUSION: Soft plaques, the presence of a lipid core and a large reduction in plaques increase the risk of microembolization during the PCI procedure.