Twenty-two patients developed thrombosis of the lower abdominal portion of the inferior vena cava, iliac or femoral vein (IVT) after cardiac catheterization. All patients has at least one previous study from 1 day to 11 months of age (mean, 2 months) and seven had two catheterizations before discovery of IVT. Transposition of the great arteries was the most common defect associated with IVT (12/22, 55%). From one to seven catheter changes were made during the time of venous cannulation (mean, 105 minutes). No clinical evidence of IVT was present immediately after the preceding cardiac catheterization. The discovery of significant IVT usually necessitated the use of a vein from the upper extremity to complete the cardiac catheterization. IVT was associated with previous balloon atrial septostomy using either a Fogarty or Rashkind septostomy catheter in 13/22 patients (59%). In addition, 86% of the patients had either of these balloon catheters or a Swan-Ganz catheter used during a previous study. The potential development of IVT should be considered especially in cyanotic infants and small children in whom balloon catheters are used.