Obtaining aprehospital 12-lead electrocardiogram (ECG) diagnostic of acute myocardial injury has been demonstrated to hasten the administration of thrombolytic agents in the emergency department. This case demonstrates that aprehospital electrocardiogram diagnostic of acute anterior wall infarction can become non-diagnostic following routine administration of oxygen, nitroglycerin, and morphine by paramedics. Although this phenomenon has been observed in the in-hospital setting, it has not been reported in patients with a prehospital ECG.