To assess extent of coronary artery injury in various types of angiographic dissection after balloon angioplasty, intravascular ultrasound studies were performed on 53 coronary segments. Angiographic dissection was classified as intraluminal filling defect (INTRALUMINAL), extraluminal extravasation of contrast media (EXTRALUMIANL), or linear luminal density (LINEAR), following the National Heart, Lung, and Blood Institute classification. Of the 14 sites with INTRALUMINAL type dissection, 2 (14%) had no obvious vessel injury, 8 (57%) had intimal injury, and 4 (29%) had intimal-medial injury with medial dissection; of the 21 sites with EXTRALUMINAL type dissection, 17 (81%) had intimal-medial injuries with medial dissection; and of the 18 sites with LINEAR type dissection, 10 (56%) had intimal-medial injuries with medial dissection. Thus coronary angiography could not adequately visualize extent of vessel injury after angioplasty, especially in the LINEAR type of dissection. Intravascular ultrasound provided a better understanding of extent of coronary artery dissection.