Recent reports have shown an exaggerated response of 17-hydroxyprogesterone in up to 70% of patients with incidentally detected adrenal adenomas ('incidentalomas'). This has been explained by pre-existing 21-hydroxylase deficiency which may be a pathogenetic factor in the development of adrenal tumours. However, other defects in steroidogenesis, such as mild 11β-hydroxylase deficiency, could also result in increased 17-hydroxyprogesterone secretion. We therefore studied the glucocorticoid and mineralocorticoid pathways in patients with adrenal 'incidentalomas' by measuring multiple adrenal steroids before and after 1–24 ACTH stimulation. Twenty patients with adrenal 'incidentalomas' (14 females, 6 males) and 27 healthy controls (14 females, 13 males) were studied. All subjects underwent a 1–24 ACTH stimulation test (250 μg i.v.) with determination of progesterone, 11-deoxycorticosterone, corticosterone, 17-hydroxyprogesterone, 11-deoxycortisol and cortisol at O and 60 min. All steroids were measured by RIA after extraction and HPLC. Patients with 'incidentalomas' had higher stimulated concentrations of 17-hydroxyprogesterone (21·6 ± 8·4 vs 4·2 ± 0·3 nmol/l; P ≤ 0·001), 11-deoxycortisol (8·1 ± 1·2 vs 3·6 ± 0·3 nmol/l; P ≤ 0·001), progesterone (8·28 ± 2·82 vs 1·08 ± 0·15 nmol/l; P ≤ 0·001), and 11-deoxycorticosterone (2·1 ± 0·39 vs 0·78 ± 0·12 nmol/l; P = 0·002) compared with controls. In contrast, cortisol and corticosterone concentrations were not different. There was evidence for impairment of 11β-hydroxylase activity by an increased 11-deoxycortisol/cortisol ratio (0·012 ± 0·003 vs 0·005 ± 0·001 in controls; P = 0·002) and 11-deoxycorticosterone/corticosterone ratio (0·04 ± 0·003 vs 0·015 ± 0·003; P = 0·003). The conclusions reached were that patients with adrenal 'incidentalomas' have increased responses of precursors of the mineralocorticoid and glucocorticoid pathway including 17-hydroxyprogesterone after stimulation with ACTH. This seems to be caused by impairment of 11β-hydroxylase activity rather than by impaired 21-hydroxylase activity in these tumours.