Over one fifth of patients hospitalized for massive pulmonary embolism die within the first few hours. The threatening load on the right ventricle can be reduced by early detection of embolism and immediate start to treatment. 53 patients treated for pulmonary embolism in our intensive care unit from 1982-1984 were divided into groups of 34 patients with massive embolism (occlusion 50% and/or circulatory shock) and 16 patients with small embolism. In 3 additional patients the embolism was detected only at autopsy. Patient's history and first clinical workup, together with special investigations and hemodynamics, allowed differentiation between massive and small embolism. Regarding treatment, patients with pulmonary embolism and circulatory shock were immediately embolectomized surgically. The other patients with massive embolism were treated by early fibrinolysis, with strict attention to any contraindications. These fibrinolyzed patients had fewer complications during the subsequent hospital course. A total of 8 patients with massive embolism and 2 patients with small embolism died during the same hospital stay. Where history and clinical findings prompt suspicion of massive pulmonary embolism a rather aggressive diagnostic procedure is indicated, since rapid reopening of the pulmonary circuit, to reduce the load on the right ventricle, may be life-saving.