A previously healthy 46-year-old male presented to our emergency department with severe thoracic pain, dyspnea and vomiting, which had suddenly started 2 hours before. He had no history of unusual features and no cardiovascular risk factors. The 12-lead electrocardiogram indicated a posterolateral myocardial infarction. Immediate coronary catheterization revealed occlusion of the proximal left circumflex artery (LCX). Recanalization and coronary stent implantation were successful. No other coronary lesions were detectable that could have indicated coronary artery disease. During catheterization, superposed intestinal loops in the left thorax were striking. The chest X-ray revealed crass cranial displacement of the left-sided diaphragm with intestinal loops beneath, leading to compression of the ipsilateral lung and to a mediastinal shift to the right. Thoracic computed tomography showed compression by the elevated diaphragm of the posterior atrioventricular groove and the left circumflex (LCX) artery embedded in this. Clinical workup revealed no muscular disorder or central dysfunction responsible for diaphragm elevation; no reason for a phrenic nerve lesion was found. The patient subsequently developed ventilatory failure, necessitating intermittent noninvasive bilevel positive airway pressure. After 5 days of intermittent ventilation, the elevation of the diaphragm reduced noticeably and respiratory assistance could be stopped. The cause of this reversible unilateral diaphragm elevation remained unknown.