Coronary angioplasty is gaining increased importance as a primary treatment of acute myocardial infarction, but the complication rate of the procedure is higher than in stable coronary artery disease. In a consecutive series of 110 coronary angioplasties in patients with acute myocardial infarction, the cause of initially failed procedures was studied by intravascular ultrasound. The balloon angioplasty was immediately successful in 66%. In those cases with crossing of the lesion but a dissatisfying dilatation result (persistent occlusion, reocclusion, or dissection) an intravascular ultrasound probe could be advanced to 32 of 34 lesions. The information provided by ultrasound guided the subsequent bailout therapy. Persistent occlusions were caused by extensive thrombosis in 4.5% of all cases, subsequently treated by local thrombolysis for 12 to 16 hours, and in 1.8% by a ruptured plaque, which was treated by stenting. In cases with Thrombolysis in Myocardial Infarction (TIMI) flow II, angiography suggested a thrombus in 9.1%, but intravascular ultrasound could detect dissections instead of a thrombus in half the cases. In cases of dissection, stenting was performed. Dissections were observed by angiography in 15.5%, and all cases were confirmed by ultrasound. In vessels >2.5 mm the dissection was treated by stenting. Overall, in 20 of 21 lesions stents were successfully implanted. No stent thrombosis was observed. With the assistance of intravascular ultrasound during bailout therapy, the success of coronary angioplasty to achieve TIMI flow III without residual stenosis in an unselected consecutive patient cohort with acute myocardial infarction was 96%. In direct angioplasty for acute myocardial infarction the procedure is frequently complicated by events such as plaque rupture and extensive vascular thrombosis, which are uncommon in coronary angioplasty for stable angina. Intravascular ultrasound provided insight into the underlying morphologic characteristics of failed angioplasty that enhanced the information provided by coronary angiography and assisted in the selection of the bailout therapy. (Am Heart J 1997; 133:517-25.)