Factors influencing the rate of post-catheterization venous thrombosis were studied in 180 infants below one year of age. The initial cardiac catheterization was performed either with cutdown technique or percutaneously. At repeat catheterization on the ipsilateral side presence or absence of thrombosis was noted. The overall thrombosis frequency was higher than previously reported, 15.6 per cent. The rate increased with decreasing weight. An increased rate of thrombosis was also found with indwelling femoral vein catheter left in place for more than 24 hours, and infection. In 6 cases, thrombosis involved only the catheterized side and would have been missed by recatheterization from the contralateral side. It was noteworthy that 9 of the thromboses spared the catheterized vessel and engaged only the vena cava. Among factors not influencing thrombosis rate were age, type of cardiac malformation, cyanosis, early operation, catheterization time or balloon septostomy. Percutaneous or cutdown technique did not influence thrombosis rate.