Coronary artery aneurysms are usually diagnosed by contrast coronary angiography, which portrays the silhouette of the lumen but cannot distinguish true and false aneurysms. To differentiate true and false aneurysms and to study the morphologic changes of the vessel wall, intravascular ultrasound (IVUS) was performed in patients with angiographic signs of coronary artery aneurysms. We used a 4.8F or 3.5F, 20 MHz IVUS catheter for ultrasound examination. Fourteen patients (12 men and two women ranging in age from 43 to 73 years) with angiographic signs of coronary aneurysm were enrolled. IVUS imaging was optimally obtained in all patients. The vessel area, lumen area, and plaque area of the aneurysm segment and of the proximal and distal segments were determined. IVUS showed that both the proximal and distal reference segments were severely affected by atherosclerotic lesions in all the patients and by calcium deposits in six patients. The percent stenoses were 63.0% ± 13.7% and 60.9% ± 17.8% in the proximal and distal reference segments, respectively. In nine patients the walls of the aneurysms showed signs of atherosclerosis. Three angiographically indicated aneurysms were found to be plaque ruptures. Although the lumen and the vessel areas of the aneurysm segments were larger than those of the proximal and distal segments (p < 0.01 and (p < 0.001), no significant differences in plaque area and plaque composition were found between the aneurysm segment and adjacent vessel segments (p > 0.05). In conclusion, IVUS allows detailed characterization of coronary aneurysms. Atherosclerosis seems to play an important role in the formation of acquired coronary aneurysms.