Nearly all patients with pulmonary embolism (PE) have complete clinical and hemodynamic and at least near complete roentgenographic and angiographic resolution within four to six weeks of the acute event. To classify the syndrome of unresolved pulmonary embolism we reviewed our experience and that of the English literature to extract 30 well-described cases. The mean age at presentation was 45 years, and most patients were less than 60 years old. Almost all complained of dyspnea, and the majority had at least one clinical event suggestive of PE. Most had clinical evidence of pulmonary hypertension. Roentgenographic, arterial blood gas and electrocardiographic findings were nonspecific, though the perfusion lung scan was always abnormal. Most patients exhibited mild-to-moderate pulmonary hypertension. The severity of pulmonary artery obstruction was quite variable and did not correlate with the magnitude of arterial hypoxemia or pulmonary hypertension. Most patients did not improve with long-term anticoagulation therapy and underwent pulmonary embolectomy with acceptable mortality. Postoperative improvement correlated with improved arterial blood gas levels, abatement of pulmonary hypertension, and more satisfactory pulmonary perfusion.