Abstract: Plasma ACTH concentrations were determined by RIA in 24 cases of Cushing’s syndrome demonstrated by operation and pathology,and were compared with the results of large dose dexamethasone suppression test.It was showed that plasma ACTH levels were elevated in 14 cases of Cushing’s disease,in high normal limit in 3 cases of Cushing’s disease at 8am,and were elevated in all cases at Oam,remarkedly decreased in all 6 cases of adrenal adenoma,and very high in one case of ectopic ACTH syndrome.The results of plasma ACTH levels showed no overlap in all the three diseases.The concordance rate with operation and pathology for plasma ACTH was 100%, while it was from 67 to 70.6% for dexamethasone suppression test.It is suggested that plasma ACTH determination is a reliable method in the differentiation of causes of Cushing’s syndrome.