Objective: Cardiac catheterization is an important and frequent diagnostic intervention in children, but few systematic studies have explored the associated venous thrombotic complications. We have prospectively evaluated the prevalence of venous thrombosis, diagnosed by ultrasonography, in children catheterized at our department. Materials and methods: We examined 50 children with weight greater than seven kilograms for thrombosis of the femoral vessels. Prophylactic anticoagulation was given to half of the children who were to undergo left-sided catheterization. The enrolled children had ultrasonography of the site of puncture the day before, and the day after, the cardiac catheterization. During catheterization, blood samples were taken from 33 of the children for analysis of thrombophilic agents. Results: The children, with a median age of 34 months, had been catheterized 103 times, comprising the 50 current and 53 previous procedures. Over the course of the current procedure, interventions of various types were performed in 18 children (36%). We diagnosed thrombophilic predispositions in four patients. Despite the thrombophilic tendencies, and a high frequency of interventions, we did not encounter a single case of femoral venous thrombosis. Nor did we find any evidence of arterial thrombosis. Conclusion: Our study indicates that catheterization procedures in children are currently associated with a low prevalence of femoral venous thrombosis. Continuous assessment of vascular complications, nonetheless, is still required to prevent late effects, and to ensure the best quality of cardiac catheterisations in children.