Sixteen cattle affected by thrombosis of the posterior vena cava were examined post mortem and the various lesions are described. The thrombus was in the hepatic portion in 12 animals and in the intrathoracic part in four. Thrombi had formed as a result of phlebitis in 13 cases in which hepatic or post-diaphragamatic abscesses were present but the aetiology of the other three was not discovered. Embolism from the vena cava thrombus resulted in pulmonary lesions in every case: these included pulmonary arterial thrombo-embolism, pulmonary arteritis, pulmonary endarteritis, chronic suppurative pneumonia and the formation of multiple pulmonary abscesses. The pulmonary arterial lesions had given rise to mycotic aneurysms of that vessel and rupture of these was followed by massive haemoptysis or intrapulmonaary haemorrhage. Pulmonary arterial aneurysms are not common in man and are very rare in animals, except in association with vena cava thrombosis of cattle. However, the "Hughes-Stovin" syndrome of man has some features in common with the condition described in cattle and these are discussed. The different sequelae of posterior vena cava thrombosis and infective endocarditis of the right heart in cattle are also considered.