OBJECTIVE This study sought to analyze the role of complex lesion morphology on the acute results of angioplasty. BACKGROUND Acute complications of angioplasty are higher in unstable than in stable angina. The unstable culprit lesion is usually complex, indicative of plaque disruption and thrombus formation. Previous nonrandomized studies have shown that the presence of intracoronary thombus increases morbidity after coronary angioplasty. The role of complex morphology in coronary angioplasty outcome was studied in a prespecified subgroup analysis of a large multicenter coronary angioplasty trial. METHODS The results of coronary angioplasty from the Thrombolysis and Angioplasty in Unstable Angina (TAUSA) trial were analyzed. This large trial randomized 469 patients in double-blinded manner to receive either intracoronary urokinase or placebo during coronary angioplasty of the culprit lesion in ischemic rest angina with or without recent infarction. The study presented here analyzes in detail the results of coronary angioplasty in complex versus simple lesions in the urokinase and placebo groups. Complex lesions were defined before angioplasty by a core laboratory as having one or more of the following: irregular borders, overhanging edges, ulcerations or intraluminal filling defects proximal or distal to the lesion. RESULTS Of the 469 patients, 458 had identifiable culprit lesions, of which 245 were complex and 213 were simple. Complex lesions were associated with a higher abrupt closure rate than simple lesions (10.6% vs. 3.3%, respectively, p < 0.003). Patients with complex lesions also had higher recurrent in-hospital angina (p < 0.02) and emergent bypass surgery (p < 0.02). Further analysis of complex lesions revealed that abrupt closure was particularly high in the urokinase group (15.0% vs 5.9% for the placebo group, p < 0.03), and most abrupt closures were thrombotic. Composite clinical end points were also significantly higher with complex lesions and urokinase. In the placebo group, complex lesions had a higher abrupt closure rate as well as postcoronary angioplasty filling defects, but clinical end points were not significantly different. CONCLUSIONS Complex lesions before coronary angioplasty increase acute complication rates after coronary angioplasty. Urokinase as administered in the TAUSA trial had significant adverse effects, especially in complex lesions. However, even in the placebo arm, complex lesions were associated with higher complication rates than simple lesions. Newer antithrombotic measures that particularly target the platelet may eventually decrease complication rates in these lesions.