The thoracic duct is the main lymphatic vessel that drains most of the body's lymph and all of the intestinal chyle into the venous system. The thoracic duct may be ligated during left radical neck dissection (RND) without any significant sequelae. The lymphatic system must, therefore, have collateral channels. The present study shows an increased incidence of pleural effusion after left RND when compared to right RND. We suggest that this represents the inability of the collateral system to immediately accommodate the increased volume of chyle.