Adrenal computed tomography with determination of Hounsfield units has proved to be sensitive and specific in the differential diagnosis of benign vs. malign adrenal lesions. On the other hand, computed tomography may fail in patients with small adrenal masses of less than 1.0 cm. However, especially in patients with diagnosed malignancies and small adrenal masses which were discovered during the diagnostic staging procedure it is important to determine the origin of the adrenal lesion. An augmented increase in 17alpha-hydroxyprogesterone (17-OHP) levels following corticotropin (1-24) stimulation has been noted in incidentally discovered adrenal masses by several groups. Therefore, we tested the hypothesis that elevated ACTH-stimulated 17-OHP (delta > 2.6 ng/mL) can predict primary adrenal lesions. We evaluated the use of the ACTH test in 85 patients with adrenocortical tumors and in 16 patients who underwent abdominal imaging for staging of a carcinoma other than of adrenal origin. We found an augmented 17-OHP response in 70 (>82%) of patients with known adrenocortical tumors and in 10 (>62%) of patients with adrenal masses and diagnosed malignancies. Results in the latter group have been confirmed in histological studies after operation or puncture. In the group of patients who suffered from a solid malignant tumor and had an adrenal mass, it was thus possible to separate primary from secondary adrenal lesions in 100%. In the group of patients with known adrenocortical tumors, it failed to differentiate between benign and malignant adrenocortical lesion in one case. We therefore think that the ACTH test is a valuable biochemical tool to distinguish primary adrenal tumors from adrenal metastasis derived from other malignancies.