A 46-year-old woman was admitted for surgery after diagnosis of a constrictive pericarditis. She had suffered from acute pericarditis and undergone pericardiocentesis to relieve pericardial effusion several times 10 years previously. Cardiac catheterization showed elevated mean right-atrial and pulmonary wedge pressures. Chest CT revealed only a calcified mass in the retrosternal space. At surgery, there was no severe adhesion between the pericardium and the epicardium, and the pericardium was thin and almost normal except over the right artio-ventricular groove, where a hard mass with an atheromatous content adhered to the epicardium. After resection of the mass, central venous pressure and pulmonary wedge pressure decreased. The mass was found to be an old hematoma by pathological examination. The patient was subsequently discharged from hospital in good condition.