Excessive limb growth and thickening is one of the clinical manifestations of congenital arteriovenous fistula, which is caused by abnormal development of the embryonic primordium during its evolution, resulting in abnormal traffic between the arteries and veins. How is congenital arteriovenous fistula diagnosed? The diagnosis is usually not difficult based on the history and physical examination. Congenital arteriovenous fistulas are associated with varicose veins. Therefore, the possibility of congenital arteriovenous fistula should be considered in young people or children when varicose veins are found without an obvious cause, especially if they are found unilaterally or in uncommon areas. The diagnosis is even more doubtful if the patient is found to have limb growth, thickening, hairiness, and sweating disorders. Arteriography is required when the decision is made for surgical treatment and to see what remains of the arteriovenous fistula after previous surgery. Arteriography can show arteriovenous traffic, but sometimes it is difficult to identify direct arteriovenous traffic because it shows many abnormal masses of vessels. Changes in the input trunk artery can be dilated and twisted due to increased blood flow; contrast buildup at the fistula; and varicose veins at the output to diagnose congenital arteriovenous fistulas. Retrograde imaging of the deep veins of the lower extremities can show regurgitation in cases of deep venous valve insufficiency. Percutaneous venous cannulation angiography can localize the function of each pair of valves in the femoral-venous system. Peripheral venous pressure measurement and PaO2 measurement can reflect regurgitant stasis and provide indirect insight into valve function and are often used as screening tests because they are simple and easy to perform. In the presence of an arteriovenous fistula, the venous pressure around the fistula is elevated and the PaO2 level in the venous blood is increased. The disease is congenital, so there are no effective preventive measures. The diagnosis requires differential diagnosis with lesions in various areas and prevention of complications. In particular, it is important to prevent the transfer of embolic material through the fistula to cause other embolic diseases.