Background: Angiographic and clinical studies have demonstrated that coronary artery plaque rupture with thrombus formation, spasm, or both are frequently responsible for the syndrome of unstable angina. Percutaneous transluminal coronary angioplasty (PTCA) is commonly used in the treatment of patients with coronary artery disease and unstable angina. A number of studies have shown, however, that intracoronary thrombus increases the risk of abrupt vessel closure. The purpose of this study was to define preprocedural variables predictive of the outcome of PTCA performed on patients with unstable angina in a prospective multicenter study using a core angiographic laboratory. Methods and Results: A total of 386 patients with unstable angina underwent coronary angioplasty of 487 lesions treated with balloon PTCA at 9 medical centers. Multivessel or left main coronary artery diseasewas present in 55% and recent myocardial infarction in 22%. Clinical success was achieved in 317 of 386 patients (82.1%), as defined by <50% residual stenosis at every target lesion evaluated in the core angiographic laboratory and no major complication during hospitalization. Major complications (death, Q-wave or non-Q-wave myocardial infarction, or emergency coronary artery bypass surgery) occurred in 36 patients (9.3%), and abrupt vessel closure occurred in 50 (13.0%). Logistic regression analysis identified preprocedural variables that were predictive of outcome of angioplasty. Strong predictors of any complication (major complication or abrupt vessel closure) included age [odds ratio (OR) = 1.04; 95% confidence interval [CII 1.02. 1.071) for each additional year of age; p < 0.001), number of diseased vessels (OR = 1.58; 95% Cl = 1.16, 2.15 per additional vessel; is = 0.012), the number of le~ions treated at angioplasty (OR) = 1.04%; 95% confidence interval [CI] 1.02, 1.07]) for each additional year of age; p < 0.001), number of diseased vessels (OR = 1.58%; 95% CI = 1.16, 2.15 per additional vessel; p = 0.012), the number of lesions treated at angioplasty (OR = 1.72%; 95% CI = 1.11, 2.66;; p = 0.014), and angiographic evidence of filling defect preceding angioplasty (OR = 3.30; 95% CI = 1.11, 9.75; p < 0.001). Conclusions: The outcome of PTCA performed for unstable angina is influenced by a combination of clinical, angiographic, and procedural variables. This study suggests that PTCA performed on lesions associated with filling defects or on more than one lesion at the time of the procedure carries an increased risk of complication. The outcome of PTCA for unstable angina may be improved by identifying new strategies for the treatment of lesions associated with filling defects and by using more accurate methods to identify and treat the culprit lesion responsible for unstable angina.