The hospital course of 144 consecutive patients with pulmonary embolism (PE) demonstrated by pulmonary angiography was reviewed to determine the mortality of patients with treated PE. Twelve patients (8%) died of PE, and eight died of causes other than PE; 124 (86%) survived. Pulmonary embolism was the primary cause of death in only four of the 12 patients who died of PE. Pulmonary embolism contributed to the death of eight other patients, each of whom had associated potentially lethal disease, particularly heart disease. The most important factor affecting mortality was shock due to acute right ventricular failure secondary to massive PE (mortality, 32%). Mortality was not related to magnitude of PE per se; the mortality of patients with massive PE without shock (6%) was the same as that for patients with submassive PE (5%. Patients with PE who survive long enough to have the diagnosis established and appropriate prophylactic therapy begun have an excellent prognosis, unless they have associated severe medical disease.