Objective　This study was To investigate the incidence rate of Cockett syndrome and relationship between Cockett syndrome and varicose veins and deep venous incompetence of the left lower extremities.Methods　73 patients(100 legs) with varicose veins of the lower extremities were investigated by descending deep venography and iliography preoperatively.Results　There were 35(47.9%) cases with abnormalities of the left common iliac vein (ALCIV)in all the cases.There were 31 cases with this conditions in the patients with varicose veins of the left or both lower extremity.While in the patients with varicose veins of the only right lower extremity,there were only 4 cases with ALCIV.In contrast,there was a significant difference between groups(χ2=9.8641,P=0.0017).In the patients with ALCIV,14 cases with Ⅲ or Ⅳ grade of deep venous incomptence were found, and only 6 cases with deep venous incompetence in the patients without ALCIV.There was a significant difference(χ2=5.3688,P=0.0205).Conclusion　Cockett's syndrome frequently occurs in varicose veins of the left lower extremity.There is a relationship between them.Cockett's syndrome might be one of the causes for the deep venous incompetence of the left leg.