We report on a rare case involving a 23-year-old female patient with mediastinal cystic mass complicated with acute pericarditis and cardiac tamponade. Pericardial fluid demonstrated lymphocyte-predominant exudate and the level of carcinoembryonic antigen (CEA) was unexpectedly elevated. Successive aspiration of mediastinal cystic mass revealed a very high level of CEA (>100,000 U/mL) and carbohydrate antigen 19-9 (>15,000 ng/mL). This patient was clinically diagnosed as an infected bronchogenic cyst complicated with pericarditis and cardiac tamponade. The treatment resulted in alleviation of her symptoms.