An aberrant placement of a central venous catheter into the upper part of the thoracic duct with loop formation in the left innominate vein was observed on catheterizing via the left internal jugular vein. The misplacement, which did not have any deleterious effects, was caused by the atypical insertion site of the thoracic duct at the dorsocaudal wall of the left innominate vein and, possibly, by its incompetent closing valve. The stiffness of the nylon catheter used may also have been a contributory factor. The possible causes of this complication of central venous catheterization are discussed. The preferential use of the right internal jugular vein is stressed.