The diagnosis of unstable angina encompasses a broad spectrum of patients with myocardial ischemia, varying widely in cause, prognosis and responsiveness to therapy. A new clinical classification of unstable angina is based on the following 2 components: severity, and the clinical setting in which unstable angina develops. The hypothesis that this clinical classification correlates with the underlying coronary artery anatomy was tested. In 238 consecutive patients, an unstable angina score ranging from 2 to 6 was determined by adding the scores for severity (1 = unstable angina without pain at rest; 2 = pain at rest > 48 hours before angiography; and 3 = pain at rest < or = 48 hours before angiographic evaluation) and the clinical setting of unstable angina (1 = unstable angina secondary to a noncardiac condition; 2 = primary unstable angina; and 3 = early postinfarction unstable angina). Fifty concurrently studied consecutive patients with stable angina were assigned a score of 0. Patients with unstable angina averaged 63 +/- 11 years of age, and 165 were men (69%). Pain at rest occurred in 202 of 238 patients (85%), and angiography was performed < or = 48 hours in 139 of these patients (69%). Among patients with unstable angina, 5 (2%) had secondary unstable angina, 143 (60%) had primary unstable angina, and 90 (38%) had postinfarction unstable angina. Multivariable regression analysis identified the unstable angina score as the most important predictor of intracoronary thrombus (p = 0.011) and lesion complexity (p = 0.004) in the ischemia-related artery.(ABSTRACT TRUNCATED AT 250 WORDS)