Unstable angina is used interchangeably with a variety of other terms to refer to a clinical situation intermediate in severity between chronic effort angina and myocardial infarction. In most reports dealing with this syndrome, the patients were selected according to a number of criteria which varied from one study to another. Some authors recognized subgroups of patients with variable severity while others looked at unstable angina as one single group. This resulted in conflicting observations and consequent dilemmas in the management of these patients. Accurate definitions are, therefore, necessary. It is proposed to divide unstable angina into two main clinical categories. Type I: This includes three subgroups. (A) Patients with known chronic angina and sudden or accelerated progression of symptoms; (B) patients with chronic angina and onset of recurrent attacks at rest; and (C) patients with angina of recent onset and rapid progression into a severe condition. Type II: (severe unstable angima). Any of the subgroups described under unstable angina Type I will qualify for this classification if the patient develops recurrent episodes of prolonged ischaemic chest pains resistant to nitroglycerin lasting for 15 minutes or more. Accurate measurement of symptoms and laboratory criteria are suggested to qualify for the different subgroups of unstable angina.