According to the diagnostic criteria for adrenal preclinical Cushing's syndrome (PreCS) established by a group headed by the Ministry of Health, Labor and Welfare (MHLW), low- and high-dose dexamethasone suppression tests (DSTs) must be performed to prove autonomous cortisol secretion, i.e., ≥3 μg/dL serum cortisol following 1-mg DST administration, and ≥1 μg/dL serum cortisol following 8-mg DST administration. However, discrepancies have been documented in the results of low-and high-dose DSTs. We therefore investigated the validity of the DST for diagnosing PreCS by performing 1-mg and 8-mg DSTs in 39 patients with adrenal incidentaloma, but no characteristic Cushingoid symptoms. In about half of these patients (20/39, 51.3%), high-dose DST was positive but low-dose was negative, and one or more of the other abnormalities of hypothalamus-pituitary-adrenal axis dysfunction was seen in 75% of these patients. Furthermore, no significant difference in incidence of glucose intolerance and hypertension was noted in patients with positive high-dose DST and negative low-dose DST compared with patients with positive low- and high-dose DST. Under the current MHLW diagnostic criteria, patients with positive high-dose DST and negative low-dose DST are not diagnosed with PreCS, but some of these patients should be. Discrepancies in the results of low- and high-dose DSTs appear attributable to the current cutoff values, and further investigations are necessary to resolve these discrepancies.