Objective To investigate the relationship between the degree of serum cortisol suppression by low-dose dexamethasone (1 mg) and full serum cortisol suppression (suppression rate > 50% ) by high-dose dexamethasone (8 mg) in patients with Cushing syndrome, and to evaluate these tests in Cushing disease. Methods Ninty-one patients with Cushing syndrome were studied retrospectively. The relationship of 20%, 30%, 40%, and 50% cortisol suppression by overnight 1mg dexamethasone with full serum cortisol suppression by overnight 8 mg dexamethasone was analyzed, and the sensitivity and specificity in the diagnosis of Cushing disease were evaluated. Results The degree of cortisel suppression during overnight 1 mg dexamethasone suppression test was correlated with that during overnight 8 mg dexamethasone suppression test (r=0. 649,P<0. 001 ). 30, 22, 13, and 9 patients had greater than 20%, 30%, 40%, and 50% serum cortisol suppression respectively during overnight 1 mg dexamethasone suppression test. Among them, 23 ( 76. 7% ), 20 (90. 9% ), 12 (92.3%), and 9 ( 100.0% )patients had full serum cortisol suppression during overnight 8 mg dexamethasone suppression test. The sensitivity of the cutoff of greater than 20%, 30%, 40%, and 50% serum cortisol suppression for the diagnosis of Cushing disease was 52.8%, 32.7%, 22.6%, and 15.7%, and the specificity was 94.7%, 94.7%, 97.4%, and 97.4% respectively. Conclusions In patients with Cushing syndrome, greater than 20% serum cortisol suppression during overnight 1 mg dexamethasone suppression test is usually associated with full serum cortisol suppression during overnight 8 mg dexamethasone suppression test, and most of them are finally diagnosed as Cushing disease.