Two cases of delayed onset hydrothorax following central venous catheterization are reported. Both involved left internal jugular vein percutaneous insertion sites, and chest roentgenograms of both demonstrated that the catheter tips were juxtaposed against the superior vena cava wall. Although both catheters functioned well for several days, we believe that this left neck site allowed for catheter tip motion, resulting in delayed perforation of the vessel. To avoid this complication we recommend that central venous catheters be checked routinely for position, with the catheter tip being parallel to the vessel wall. The left neck approach should be avoided if possible because of anatomic and mobility problems. These cases emphasize the fact that hydrothorax can occur long after successful catheter insertion, and they demonstrate the need to have continued suspicion of the possibility of this occurrence.