In seven cases with pleural effusion transosseal azygography has been per formed, in order to specify any cause for the fluid accumulation that could be related to the azygous system. In 4 cases azygography appeared normal and further laboratory work-up revealed collagen disease, heart failure and lung cancer. In the remaining 3 cases (2 with recent myocardial infarction and 1 with unstable angina) azygography discerned thrombosis of the hemiazygous vein. It is suggested that in cases with an otherwise unexplained left-sited thoracic tran sudate a thrombosis of the hemiazygous vein may be visualized by transosseal azygography.