The prevalence of right-sided cardiac mobile thrombi, "in transit" from the systemic venous system, was 18% in a series of 130 patients with massive pulmonary embolism referred to early echocardiography and receiving thrombolytic drugs (56%) or intravenous heparin (40%). The mortality rate was lower than previously reported and seemed to be related more to clinical and hemodynamic impairment than to presence of thromboembolus.