Venous thrombosis is the development of a blood clot in a vein. Post-thrombotic syndrome (PTS) is a painful condition that can develop following a venous thrombosis in one of the deep veins of the leg. While PTS is mainly thought to occur because of damage to the vein, other factors may be responsible for the development of this condition. This study will analyze genetic and biologic samples from participants of a previous study to examine other possible causes of venous diseases and PTS.