Venous aneurysm not associated with other cardiovascular disease or trauma is a rare condition. A case is presented of bilateral jugular and mediastinal venous aneurysm, which was detected and depicted to a full extent by means of a blood-pool study with 99mTc -fibrinogen, undertaken because of concurrent thrombocytopenia. The aneurysm was confirmed by subsequent X-ray angiography and surgery. In radioisotopic blood pool study venous aneurysm, unlike arterial or aortic aneurysm, may be filled to the equilibrium state later than the appearance of the cardiac pool.