Superior vena cava syndrome is a clinically well-recognized entity cau
sed by malignant mediastinal neoplasms in 80% of instances. A rare ben
ign cause of the superior vena cava syndrome due to a retrosternal thy
roid is presented. This case illustrates that it is the site of the ma
ss and the completeness of obstruction, rather than the actual size of
the mass, that determine the extent of collaterals that develop.