In a patient with clinical and hemodynamic criteria of cardiac tamponade, during the acute phase of myocardial infarction, a two dimensional echocardiographic study showed pericardial effusion with an echo-dense mass in the pericardial space. Subacute ventricular free-wall rupture diagnosis was suspected. A cardiac computerized tomography (CT) and magnetic resonance (MR) study was made. CT showed an elevated density (32 HU) of pericardial effusion suggesting hemopericardium. RM imaging showed a very high and homogeneous signal in the pericardial space consistent with a methemoglobin phase clot. Anatomic confirmation was not possible.