Recurrent pulmonary embolism sometimes (3% of hospital autopsies) determines a progressive obstruction of the pulmonary vascular bed, which in turn causes pulmonary arterial hypertension and in time right ventricular hypertrophy and failure. The first stages of this process are characterized by slight pulmonary arterial hypertension at rest and by few and deceiving symptoms which make the diagnosis very difficult. Regarding anatomy, in most cases recurrent thromboembolism obstructs one of the main branches of the pulmonary artery. At the beginning pulmonary embolism usually manifests itself in a spontaneous and atypical manner: paroxysmal dyspnea, tachycardia, lateral chest pain, mild hemoptysis and recurrent fever. The clinical signs of peripheral thrombophlebitis are not very frequent. The chest roentgenogram supplies diagnostic information in 20% of cases, the electrocardiogram in 10%. Very important is the contribution of the analysis of arterial blood gases: hyperventilation, moderate hypoxia associated with shunting, hypocapnia with a widened difference between alveolar and arterial CO2. Pulmonary perfusion scintiphotography shows vast unperfused areas, different to the "plexogenic" appearance in primitive pulmonary arterial hypertension, in about 50% of cases. Pulmonary angiography discloses the exact site and extension of the obstruction in 80-90% of cases. On catheterization pulmonary arterial hypertension results to be inconstant and may appear only during stress. Regarding the evolution of pulmonary embolism, the forms associated with pulmonary arterial hypertension may last several years, although recurrent embolism may shorten its course. When the stage of right ventricular hypertrophy is reached, the evolution is generally rapid (from 1 to 4 years).(ABSTRACT TRUNCATED AT 250 WORDS)