Objective: An increased response of 17-hydroxyprogesterone to ACTH stimulation has been observed in adrenal incidentaloma and linked to an impairment of either 21-hydroxylase or of 11b-hydroxylase activity. To analyse this question further, we investigated the steroidogenic pathways in a series of 17 adrenal incidentalomas. Design and Patients: 17 patients (7 women, 10 men; mean age, 62 Ϯ 12 years) with non-histologically analyzed adrenal incidentalomas were prospectively evaluated. Methods:The following variables were investigated: 24-h urinary methanephrines and free cortisol excretion; plasma levels of ACTH and dehydroepiandrosterone; overnight dexamethasone suppression test; 1-24 ACTH stimulation test with measurement of: cortisol, 11-deoxycortisol, 17-hydroxyprogesterone, aldosterone, 11-deoxycorticosterone, progesterone, 17-hydroxypregnenolone, D4-androstenedione, dehydroepiandrosterone and 21-deoxycortisol. Results: Discordant features of subclinical hypercorticism were noted in one case. No patient had dehydroepiandrosterone sulfate levels in the normal range for his or her age. Peak 17-hydroxyprogesterone and peak 21-deoxycortisol disclosed impairment of 21-hydroxylase in 11 and 10 cases respectively. An increased 11-deoxycortisol/cortisol ratio identified reduced activity of 11b-hydroxylase in 11 patients. Eight patients displayed features of mild 17,20-lyase impairment, which was related to 21-hydroxylase dysfunction. Whereas only 2 patients showed no enzyme modification, 9 displayed alterations of at least two pathways. Conclusion: In our hands, a combination of enzyme dysfunction was frequently observed. Shared biochemical mechanisms could explain combined 17,20-lyase and 21-hydroxylase alterations, whereas coexistence of 21-hydroxylase (particularly when based on peak 21-deoxycortisol) and 11b-hydroxylase is more puzzling.