Objectives: We used serial intravascular ultrasound (IVUS) to assess the efficacy of rosuvastatin on plaque regression in patients with mild to moderate degree of coronary stenosis (stenosis of 30 to 60% by QCA) with vulnerable plaque (plaque with large lipid core with thin fibrous cap). Methods: This study was a prospective, randomized, single center study for lipid lowering therapy using rosuvastatin 20 mg or atorvastatin 40 mg. IVUS was performed during baseline catheterization and repeated after 12 months of treatment. Efficacy parameters included changes in atheroma volume and the lipid-rich pool size determined by IVUS. A total of 31 lesions in 17 patients (rosuvastatin group: 17 lesions in 10 patients vs. atorvastatin group: 14 lesions in 7 patients) were analyzed. Results: LDL-cholesterol levels were reduced from 120짹40 mg/dl to 70짹28 mg/dl in rosuvastatin group (p<0.001) and from 128짹46 mg/dl to 72짹33 mg/dl in atorvastatin group (p<0.001). HDL-cholesterol levels increased 11% with rosuvastatin (p<0.001) and 10% with atorvastatin (p<0.001). High-sensitivity C-reactive protein decreased 37% with rosuvastatin (p<0.001) and 35% with atorvastatin (p<0.001). Lumen volume increased and vessel volume and atheroma volume decreased at follow-up in both groups (rosuvastatin group: lumen volume, 313짹84 to 315짹91 mm3, p<0.001; vessel volume, 581짹131 to 578짹138 mm3, p<0.001; and atheroma volume, 268짹65 to 263짹76 mm3, p<0.001, and atorvastatin group: lumen volume, 333짹91 to 334짹82 mm3, p<0.001; vessel volume, 656짹182 to 654짹193 mm3, p<0.001; and atheroma volume, 323짹104 to 319짹93 mm3, p<0.001). The change of percent atheroma volume [100��plaque volume/EEM volume] was -7짹1% in rosuvastatin group and -3짹1% in atorvastatin group (p=0.257). Lipid-rich pool size was reduced in both rosuvastatin (0.9짹0.4 mm2 to 0.2짹0.2 mm2, p<0.001) and atorvastatin group (0.8짹0.2 mm2 to 0.3짹0.2 mm2, p<0.001). Conclusions: Therapy with rosuvastatin 20 mg showed similar effects in regression of coronary atherosclerosis compared with atorvastatin 40 mg. Both rosuvastatin 20 mg and atorvastatin 40 mg could contribute to the regression of lipid-rich plaque and to the stability of coronary plaque.