The lower extremities of 28 unselected children with congenital heart disease were investigated and classified according to the criteria for postthrombotic syndrome five to ten years after their first cardiac catheterization. For the clinical criteria, all patients completed a questionnaire and underwent a standardized physical examination of both legs. For the pathophysiologic criteria, the presence of venous outflow obstruction and reflux was evaluated by color duplex sonography in 24 of the 28 patients. Mild postthrombotic syndrome was present in half the patients. Partial or complete occlusion of the investigated vein was found in four patients (17%). In all patients studied, the venous valves of the deep system were competent. Postthrombotic syndrome frequently occurs in children with congenital heart disease. Prospective studies seem to be justified to investigate the precise incidence and potential risk factors.