Technetium-99m-phosphate imaging is particularly valuable in detecting (1) small transmural infarcts (3 g and larger in size); (2) new acute transmural infarcts in or near regions of old infarction; (3) acute subendocardial infarcts (larger than 3 g in size); (4) acute infarction in patients with left bundle branch block; and (5) perioperative myocardial infarction. Localization of inferior and posterior myocardial infarction is improved with imaging. Sizing of acute anterior and lateral infarcts has been accurately done in dogs and should prove helpful in patients. Extensive evaluation in both experimental animals and in patients has shown 99mTc-phosphate myocardial imaging to be a useful clinical tool, and it may be one of the most sensitive noninvasive ways presently available to identify acute myocardial necrosis. It is important to understand that 99mTc-phosphate imaging has a different pathophysiology basis from EKG's or serum enzymes. These tests do not compete but instead should complement one another.