Patients who undergo elective percutaneous transluminal coronary angioplasty (PTCA) provide a unique human model, with fully defined coronary anatomy and known sites and periods of coronary occlusion. High-resolution, quantitative ST segment monitoring during PTCA procedures allows characterization of the first moments of coronary occlusion, as well as the effect of interventions intended to reduce ischemia during occlusion. Data from more than 1,000 coronary occlusions are reviewed in this presentation. Intracoronary interventions during acute myocardial infarction (MI) and elective PTCA create a potentially unstable coronary nidus through their therapeutic effect. Angiographic evidence of the anatomic appearance remains the "gold standard" for real-time assessment of success although angiography has limited ability to identify the 2-4% of patients who suffer morbidity or mortality in the coronary care unit (CCU) after an apparently successful intervention. Standard CCU monitoring of patient symptoms and rhythms underestimates transient ischemic activity in 87% of patients who experience ischemia after interventions. The capabilities of real-time multilead ST segment monitoring are reviewed from 8,331 hours of monitoring in 338 patients after intracoronary interventions. The documentation of persistent ischemia after angiographically successful PTCA and the clinical potential of qualitative patient-specific, coronary site-specific, precordial "fingerprinting" for identification of high-risk patients before or in the absence of anginal symptoms are discussed.