We present the case of a 20-year-old man who was stabbed in the left chest and was diagnosed with a large pericardial effusion by focused emergency department (ED) ultrasonography. After placement of a left chest tube for presumed tension pneumothorax, the pericardial effusion had resolved. The patient's postinjury course was complicated by pericarditis and recurrent tamponade, which required repeated pericardiocentesis for management. This case illustrates the role of focused ED ultrasonography for diagnosis of pericardial effusion in penetrating trauma and the potential for delayed pericardial effusion and tamponade in such patients. Although the pathophysiology of delayed pericardial effusion is unclear, autoimmune postpericardiotomy syndrome has been proposed as the cause of this rare condition. Our case underscores the importance of close monitoring of patients with known or suspected pericardial injuries due to their potential for the development of lifethreatening complications.