Renal artery thrombosis is rarely encountered in the pediatric population. The recent literature associates its occurrence with the use of umbilical artery catheterization. This report describes renal artery occlusion in 2 patients with severe cardiac disease who developed renal artery thrombosis a few weeks after birth. One patient had undergone umbilical artery catheterization months before he developed the renal artery occlusion. The second patient never had umbilical artery catheterization. Neither patient recovered renal function in the affected kidney despite systemic anticoagulation.