A 44-year-old man presented with symptoms and signs of cardiac tamponade. Cytologic examination of the pericardial fluid was negative for malignancy and no manifestations of primary tumor were detected. Two weeks after pericardiocentesis the patient developed constrictive pericarditis. An emergency exploratory thoracotomy revealed a thick, fibrotic pericardium firmly adherent to the underlying myocardium. Histologic examination of the pericardium showed the presence of an adenocarcinoma, suspected to be a metastatic dissemination from a primary pulmonary source. The lymphatic spread of the tumor to the heart may explain the early development of pericardial effusion without malignant cells and the later development of pericardial and epicardial thickening. Cardiac tamponade of unknown origin should prompt a search for metastatic carcinoma, including in presence of a negative cytology.