This study investigated whether vascular damage and quantitative changes observed with intravascular ultrasound at the most stenotic site are representative of the ultimate outcome after coronary balloon angioplasty. Atherosclerotic coronary arteries (n = 40) were studied in vitro with intravascular ultrasound. From each vascular specimen, 10 corresponding intravascular ultrasound cross sections obtained before and after balloon angioplasty were selected for comparison with their histologic counterpart. Morphologic and quantitative data obtained from all cross sections were compared with data derived from the most stenotic site. The incidence of vascular damage (i.e., dissection, plaque rupture, and media rupture) at the most stenotic site was lower than that seen for each vascular specimen. The sensitivity of intravascular ultrasound in detecting these morphologic features for each vascular specimen was high for dissection and media rupture (79% and 76%, respectively), and low for plaque rupture (37%). After balloon angioplasty, quantitative changes seen at the most stenotic site were greater than those in all cross sections: free lumen area +58% versus +29%, media-bound area +17% versus +12%, and plaque area reduction -9% versus -6%, respectively. The increase in free lumen area was caused predominantly by media-bound area increase (81%) and to a lesser extent by plaque area decrease (19%). This study revealed that a higher incidence of vascular damage is found when the whole segment is analyzed rather than 1 single cross section at the most stenotic site. Quantitative effects of coronary balloon angioplasty seen with intravascular ultrasound were greater at the most stenotic site than at all cross sections.