Once the results of creatine kinase measurements and serial electrocardiograms are available, the identification of patients experiencing non-Q-wave myocardial infarction is relatively simple. Identification of patients experiencing unstable angina is more difficult, since numerous definitions of the condition have been offered. There is agreement, however, on two important features of unstable angina. First, ischemia usually develops at rest or is precipitated by minimal exertion; this differs from chronic stable angina, in which most ischemic episodes are precipitated by physical exertion or strong emotion and the resultant increase in myocardial oxygen demand. Second, ischemia is often associated with transient ST-segment depression or elevation, in contrast to the persistent ST-segment elevation characteristic of patients who develop Q-wave infarction.