OBJECTIVES: The aim of this study was to examine the frequency of Cushing's syndrome (CS) in obese patients devoid of specific clinical symptoms of Cushing's syndrome. METHODS: A total of 150 obese patients (129 female, 21 male; mean age 44.41 ± 13.34 yr; mean BMI 35.76 ± 7.13) were included in the study. As a first screening step, we measured 24-h urinary free cortisol (UFC). An overnight 1-mg dexamethasone suppression test was also performed on all patients. Urinary free cortisol levels above 100 μg/24 h were considered to be abnormal. Suppression of serum cortisol <1.8 μg/dL after administration of 1 mg dexamethasone was the cut-off point for normal suppression. The suppression of the serum cortisol levels failed in all of the patients. RESULTS: Measured laboratory values were as follows: ACTH, median level 28 pg/ml, interquartile range (IQR) 14-59 pg/ml; fasting glucose, 100 (91-113) mg/dL; insulin, 15.7 (7.57-24.45) mU/ml; fT4, 1.17 (1.05-1.4) ng/dL; TSH, 1.70 (0.91-2.90) mIU/L; total cholesterol, 209 (170.5-250) mg/dL; LDL-c, 136 (97.7-163) mg/dL; HDL-c, 44 (37.25-50.75) mg/dL; VLDL-c, 24 (17-36) mg/dL; triglycerides, 120.5 (86-165) mg/dL. The median UFC level of the patients was 30 μg/24 h (IQR 16-103). High levels of UFC (>100 μg/24 h) were recorded in 37 patients (24%). Cushing's syndrome was diagnosed in 14 of the 150 patients (9.33%). Etiologic reasons for Cushing's syndrome were pituitary microadenoma (9 patients), adrenocortical adenoma (3 patients), and adrenocortical carcinoma (1 patient). CONCLUSION: A significant proportion (9.33%) of patients with simple obesity were found to have Cushing's syndrome. These findings argue that obese patients should be routinely screened for Cushing's syndrome.