Cushing's syndrome, including its mild form/state of adrenal-dependent subset (subclinical Cushing's syndrome; subCS), is known to enhance glucose intolerance, hypertension and obesity. Recently, subclinical Cushing's disease (subCD) has been identified, but its prevalence and the extent of consequent metabolic derangement are unclear. We screened 90 type 2 diabetic patients hospitalized in our department for subCD, according to the diagnostic guideline proposed by the working group of Japanese Ministry of Health, Welfare and Labor in 2006. Plasma ACTH and cortisol levels in the morning and at midnight were determined, and overnight 0.5 mg dexamethasone suppression test (DST) was performed. Those who showed poor cortisol suppression in DST underwent the desmopressin (DDAVP) test. Fifty-seven patients (63.3%) demonstrated abnormally high midnight cortisol levels (>or=2.5 microg/dL), while only nine of them failed to suppress plasma cortisol levels to <3 microg/dL after DST. Although none of the eight patients who underwent the DDAVP test demonstrated the anticipated paradoxical rise in plasma ACTH, these eight patients (8.9%) endocrinologically met the screening criteria of subCD. Since a considerable percentage of pituitary adenomas causing overt Cushing's disease are not identifiable in magnetic resonance imaging, many of those causing subCD may also be unidentifiable. Further follow-up studies including confirmatory testing and pituitary imaging are necessary.