A prehospital 12-lead electrocardiogram (ECG) is commonly used for patients with suspected ST-segment elevation myocardial infarction (STEMI). This case report describes how paramedics diagnosed inferior STEMI with all ECG leads positioned on a patient's back (i.e., "all-posterior" positioning). The patient was hemodynamically stable but morbidly obese and markedly diaphoretic. Owing to severe back pain, he refused to lie in the supine position for assessment or transport. At the emergency department, a 12-lead ECG with the patient in lateral recumbency confirmed the diagnosis of inferior STEMI. This case shows that an all-posterior 12-lead ECG can be used to identify STEMI when optimal patient positioning is not possible.