Unstable angina is a simple term used to describe a complex group of conditions with a heterogeneous pathogenesis and prognosis. In patients with cardiac disease, understanding pathogenetic mechanisms often influences decisions regarding prognosis and treatment. Potential causes for the development of acute myocardial ischemia include: 1. Extracardiac factors in the patient with severe coronary atherosclerosis. 2. Plaque disruption resulting in: a. Transient platelet aggregation in diseased vessels. b. Dynamic or intermittent coronary artery thrombosis. c. Hemorrhagic dissection into an atheromatous plaque. d. Abnormal constriction of a coronary artery. 3. Progression of atherosclerosis as a result of plaque "healing." It may not be possible to identify the appropriate mechanism responsible for unstable angina in every case but the clinician must attempt to do so since the selection of appropriate therapy for the individual patient depends on the mechanisms responsible for the symptoms.