Klippel-Trenaunay (KT) syndrome is a rare, sporadic, congenital vascular disease of unknown etiology. We describe pulmonary findings in an 18-year-old male patient followed up since birth with the KT syndrome. The patient developed pleural and pericardial serous effusions that led to an open lung biopsy. Previous pulmonary findings have been limited to thromboembolic phenomena and pulmonary vein varicosities. On the other hand, reports of lymphatic hyperplasia, aplasia, and hypoplasia in KT have been limited to the extremities. For the first time, we describe lymphatic involvement of the lung in KT. The plexiform hyperplasia of the lymphatic channels with smooth muscle hyperplasia leading to lymphatic obstruction, pleural and pericardial effusions are new findings. The lymphatic nature of the plexiform channels was confirmed by immunohistochemistry. Von Willebrand factor and QD-END/10 monoclonal antibodies either did not react or reacted poorly with lymphatic endothelium, features used to distinguish lymphatic and venous endothelium. Ultrastructurally, the absence of basement membrane continuity further substantiated the lymphatic nature of the channels. From our findings, the lymphatic abnormality in the syndrome appears to be more generalized than previously thought. This entity should be distinguished from lymphangioleiomyomatosis to which it bears a superficial morphologic appearance.