OBJECTIVE: To investigate the possibility of a different prevalence of subclinical Cushing's syndrome (SCS) and potentially related morbidities between patients with unilateral adrenal incidentalomas (UAI) and bilateral adrenal incidentalomas (BAI), as existing data are few and controversial.DESIGN: Prospective observational study.METHODS: Clinical examination, biochemical tests, and hormonal evaluation were performed in 298 consecutive patients with adrenal incidentalomas, unilateral in 224 patients (75.2%), bilateral in 74 patients (24.8%), with apparently benign masses based on imaging characteristics and after exclusion of overt endocrine disease. The diagnosis of SCS was based on a post-dexamethasone suppression test (2 mg dexamethasone/24 h for 48 h), with serum cortisol level ≥1.8 μg/dl combined with at least one abnormal result of the other hormonal measurements.RESULTS: SCS was diagnosed in 66 out of 298 (22.1%) patients, being more frequent in patients with BAI (35.1 vs 17.9%, P=0.003, for BAI and UAI respectively). Hypertension, type 2 diabetes mellitus, impaired glucose tolerance, and dyslipidemia were of a similar frequency in both groups. SCS patients with UAI and BAI did not differ in age, gender, BMI, waist circumference, and mass size. Factors related to SCS were the presence of BAI (OR, 3.24; 95% CI, 2.31-4.54) and mass size (OR, 2.63; 95% CI, 1.31-5.26).CONCLUSION: BAI patients present more often with SCS when compared with UAI patients; however, morbidities potentially related to subtle cortisol hypersecretion were of a similar frequency in both groups. Further studies are needed to clarify whether this difference in hormonal activity may be related to different pathophysiologies.