Objective To evaluate the values of bilateral inferior petrosal sinus sampling ( BIPSS),high dosage dexamethasone suppression test (HDDST) and pituitary image in the differential diagnosis of ACTH-dependent Cushing's syndrome. Methods Totally 87 patients with confirmed pathological diagnosis were recruited in the study.All received the procedure of BLPSS,HDDST,and pituitary MRI with dynamic enhancement.The diagnostic performances of three differential diagnosis methods in ACTH-dependent Cushing's syndrome were evaluated.Results Seventy-eight patients were diagnosed as cases of pituitary ACTH adenoma,and the remaining 9 were confirmed cases of ectopic ACTH syndrome due to the thymic carcinoid.The sensitivity and specificity of HDDST,pituitary MRI,and BIPSS for the diagnosis of ACTH-dependent Cushing's syndrome were 82.1％ and 100％,79.5％ and 44.4％,92.3％ and 100％,respectively.In Cushing's disease,the diagnostic accuracy was 83.9％ with HDDST,77.0％with pituitary MRI,and93.1％ with BIPSS.In those patients with Cushing's disease,the coincidence of lateralization was 83.9％ with BIPSS and 64.5％ with MRII.Conclusion BIPSS was better than the other two methods in differential diagnosis of ACTH-dependent Cushing's syndrome.Compared with the pituitary MRI,the concordant rate of BIPSS in lateralization of the tumor is higher,and it is more reliable.