The establishment of a prognosis and the approach to the treatment of many diseases is aided greatly by a logical classification. For example, classification of a wide variety of neoplasms by anatomic extent, microscopic appearance, and the presence of special markers now forms the basis for selecting appropriate therapy. In cardiology, the classification of patients with acute myocardial infarction and congestive heart failure has been of enormous value in following the progress and in selecting therapy of individual patients and in comparing the outcome of similar patients treated at different locations and at different times. The purpose of this article is to provide a classification of unstable angina. This classification is designed to facilitate communication about these patients, to aid in the decision regarding diagnostic measures and therapy of individual patients, and to provide a more precise basis for including patients in and for evaluating the outcome of clinical trials.