Neurofibromatosis type I (NF-1) is the most common eurocutaneous syndrome. Associated vascular abnormalities are arterial occlusion, aneurysm, ectasia and arteriovenous malformation. Spontaneous massive hemothorax due to rupture of an arterial aneurysm is rare but fatal. It is, therefore, essential to determine the location of an aneurysm and provide immediate surgical or other interventional treatment. We report a case of spontaneous hemothorax caused by rupture of an intercostal arterial aneurysm diagnosed by CT and angiography.