Pericardial effusion, a common cardiac manifestation of myxedema,1,2is rarely associated with cardiac tamponade; to our knowledge, only 19 reports of this phenomenon appear in the English literature.3-7In all reported cases, identification of tamponade was largely derived from clinical and echocardiographic examination, and available hemodynamic data were described in only one report. In this presentation, the hemodynamic measurements relevant to the diagnosis of cardiac tamponade and the dynamic physiologic alterations subsequent to pericardiocentesis are more fully elucidated by reference to the recently described hemodynamic concept of pericardial constraint.8,9 REPORT OF A CASE A 63-year-old obese man was admitted to the coronary care unit because of increasing dyspnea and swollen legs over the preceding several weeks. Digitalis and furosemide had been prescribed for presumed heart failure. At the time of hospital admission, the patient had overt tachypnea, respiratory distress, and cyanosis. Blood pressure was 100/60 mm Hg,