A 67-year-old female was admitted due to dyspnea. A computed tomography (CT) disclosed a suspicious pulmonary mass and a pericardial effusion. A transthoracic echocardiogram confirmed a large-volume circumferential pericardial effusion. A pericardiocentesis was performed, and the cytological and histochemical studies later confirmed the diagnosis of pulmonary adenocarcinoma. This case report highlights the casualty of having found a cardiac tamponade through a CT not synchronized with an electrocardiogram.