Patients with non-hyperfunctioning adrenal adenomas often have an increased plasma 17-hydroxyprogesterone response to ACTH stimulation. The effects of adrenal surgery on this abnormality have rarely been investigated. One hundred and sixty-one patients with unilateral adrenal tumors (non-hyperfunctioning adenomas, 78; cortisol-producing adenomas, 8; aldosterone-producing adenomas, 37; adrenal cysts, 12; pheochromocytomas, 26) were studied. Patients before and after adrenal surgery as well as 60 healthy subjects underwent an ACTH stimulation test using 2 mg synthetic ACTH(1-24) (Cortrosyn Depot, Organon). Basal and ACTH-stimulated plasma 17-hydroxyprogesterone and cortisol concentrations are reported. Before adrenal surgery, the basal plasma 17-hydroxyprogesterone concentrations were normal in patients with all types of tumors. However, the ACTH-stimulated plasma 17-hydroxyprogesterone levels were abnormally increased in 53% and 31% of patients with non-hyperfunctioning adenomas and aldosterone-producing adenomas, respectively. In addition, a few patients with adrenal cysts and pheochromocytomas also showed an increased ACTH-stimulated 17-hydroxyprogesterone response. After unilateral adrenalectomy, this hormonal abnormality disappeared in most, although not all patients with adrenal tumors. In patients with non-hyperfunctioning adrenal tumors, ACTH-stimulated plasma 17-hydroxyprogesterone and cortisol concentrations significantly correlated with the size of the tumors. These results firmly indicate that the tumoral mass itself may be responsible for the increased plasma 17-hydroxyprogesterone and cortisol responses after ACTH stimulation in patients with non-hyperfunctioning and hyperfunctioning adrenal adenomas.