Because, in recent years, patients with incidentally discovered adrenal masses have been encountered increasingly, their endocrine function was investigated in basal conditions and after dynamic tests. Thirty-two patients (23 women and 9 men, aged 28-74 years) were studied. Lesion diameter, as documented by computed tomography and/or nuclear magnetic resonance imaging, ranged between 5 and 65 mm; the tumors were localized on the right in 22 patients, on the left in 5 and bilaterally in 5 cases. In basal conditions, urinary free cortisol (UFC) excretion, plasma adrenocorticotropin (ACTH) and cortisol levels were normal, except for 4 patients who showed high UFC and ACTH levels in the low-normal range. Ovine corticotropin-releasing hormone (CRH, 1 microgram/kg iv) was given to 18 patients, inducing normal ACTH and cortisol responses in 12, blunted responses in 4 and no response in 2 cases. No reduction in ACTH and cortisol levels after suppression tests was observed in 4 of 29 patients after dexamethasone (1 mg overnight) or in 6 of 29 after loperamide. The 4 patients who were unresponsive to both tests did not show any further inhibition after high-dose dexamethasone administration, had low plasma ACTH levels and showed impaired or absent responses to the CRH test: they were diagnosed as affected with preclinical Cushing's syndrome. An exogenous ACTH test performed in 30 patients caused a normal cortisol rise. Basal mean 17-hydroxy-progesterone (17-OHP) levels were not different from those in normal subjects.(ABSTRACT TRUNCATED AT 250 WORDS)