Objective: Recent studies suggest a possible connection between silent adrenal nodules and mild forms of 21-hydroxylase deficiency. It remains unclear whether the enzymatic deficiency is generalized or intrinsic to the adrenal mass. To help to clarify this, we have studied 17 alpha-OH-progesterone (17OH-P) response to ACTH stimulation in a group of patients with adrenal 'incidentaloma' in comparison with normal subjects. In patients who underwent surgical treatment, the test was repeated to evaluate possible modifications in 17OH-P behaviour after resection of the adrenal mass., Subjects and Methods: Fifteen subjects with incidentally discovered asymptomatic adrenal masses were studied. Basal hormone evaluations were normal, with normal cortisol suppression after low-dose dexamethasone. Iodocholesterol scanning, performed in 12 patients, showed normal bilateral adrenal uptake in 2 subjects and an increased uptake on the side of the lesion in 10 subjects. In every patient, ACTH stimulation was performed to evaluate the secretory response of cortisol, 17OH-P, progesterone and dehydroepiandrosterone sulphate. An identical test was performed in 10 control subjects with normal adrenal glands, matched for age and sex. In six patients with an adrenal lesion > 3.5 cm, the ACTH stimulation test was repeated one month after surgery., Results: The 17OH-P response to ACTH stimulation was significantly higher in subjects with adrenal 'incidentaloma' than in controls (P < 0.01). In particular, 10 subjects out of 15 (66%) evidenced a 17OH-P peak > 18 nmol/l at 60 minutes. No differences were seen in baseline 17OH-P or cortisol levels or in cortisol response to ACTH between the two groups. Dehydroepiandrosterone sulphate concentrations were significantly lower in patients with adrenal 'incidentaloma' than in normals. In six patients who had an increased 17OH-P response to ACTH on initial evaluation, the ACTH test was repeated one month after surgery. In five of these patients, 17OH-P response to ACTH was clearly reduced, suggesting that in these cases the enzymatic defect was restricted to the adenoma. In the other patient, however, stimulated 17OH-P levels remained unchanged. In this case, therefore, all of the adrenal tissue seems to be involved, suggesting a late-onset 21-hydroxylase deficiency. No significant modifications in cortisol response to ACTH were observed., Conclusion: It seems therefore that in some cases of incidentaloma the steroidogenic enzymatic defect may be secondary to the adenomatous proliferation, while in others such defects may induce the development of silent adrenal nodules.