The incidence of complications following treatment of acute appendicitis in childhood is lower than 5%, while thromboembolic disease at that age is exceptional. The case of a 13-year-old male who was operated on in our pediatric surgery department for acute gangrenous appendicitis is presented. His condition improved until the sixth day after the operation when he suffered thrombosis in both the left iliac vein and the inferior cava. Ultrasonography, plethysmography and computer tomography (CT) studies were carried out and a diagnosis of venous thrombosis was made with an underlying congenital vascular malformation where the iliac vein was compressed by the overlying iliac artery; known as the Cocket syndrome. The etiology diagnosis and treatment of this rare disease in childhood is reviewed.