Twenty-four patients with subacute massive pulmonary embolism were studied both during their initial illness and up to nine years after it. The most common mode of presentation was progressive dyspnoea over a two to 12 week period, which in some, but not all, patients was accompanied by pleuritic chest pain and haemoptysis. Physical signs at diagnosis usually suggested right heart strain and ventilation/perfusion mismatch and in the five patients with the highest pulmonary artery pressures the pulmonary component of the second sound was accentuated. The chest x-ray and electrocardiogram provided useful diagnostic information in most patients though occasionally they were normal. Early response to thrombolytic treatment was poor when compared with patients with acute pulmonary embolism but was occasionally dramatically successful, and heparin alone provided satisfactory treatment in the eight patients receiving it. Pulmonary embolectomy provided poor results and four of the five patients undergoing this form of treatment died. Nine patients died during the initial illness and in seven death was directly related to embolic disease. One patient died from neoplastic disease during follow-up. Though the prolonged illness, poor initial response to treatment, and absence of predisposing factors suggest that recurrent embolic disease and late pulmonary hypertension might occur three was no evidence of this during a follow-up period of one to nine years (median five years).