Adrenal masses are discovered incidentally in 1 to 3% of abdominal CT scans performed for investigation of nonadrenal-related abdominal complaints. Although most incidentally discovered masses are clinically silent, 10% are hormone-secreting and are associated with subtle symptoms of hormone excess. The major concern is the possibility that such a mass is malignant and requires surgical intervention. Benign adrenal cortical adenomas are 60 times more common than primary adrenal cortical carcinomas, which are rare, and many of the lesions that are malignant are metastatic from extra-adrenal neoplasms. Size is a significant factor in determining the probability that the lesion is benign or malignant, and there is concensus that most benign lesions are smaller than 3 cm, whereas most malignant lesions are larger than 6 cm. Uncertainty remains, however, as to the potential malignant character of masses measuring 3 to 6 cm; thus, size alone is insufficient for determining if an incidentally-found adrenal mass is benign or malignant, and information obtained from other diagnostic techniques, such as CT, MR imaging, and adrenal scintigraphy with 131I 6 beta-iodomethylnorcholesterol, should be considered. Fine needle biopsy of an adrenal mass is not recommended unless there is strong suspicion that the mass is metastatic from an extra-adrenal neoplasm.