The presence of an androgen-secreting tumor in a 29-year-old woman was confirmed and its location was determined by computerized axial tomographic (CAT) scanning. The hormone production from this virilizing adrenal adenoma was studied in vivo and in vitro. The major secretory products of the tumor (as compared to normal adrenal tissue) were testosterone (24-fold) and 17 beta-estradiol (five-fold). Although the adenoma produced lesser amounts of dehydroepiandrosterone sulfate (DHEAS), the demonstration of elevated serum testosterone and DHEAS in serial samples was a better marker for an androgen-secreting adrenal tumor than were the urinary 17-ketosteroids, which remained in the upper limit of normal. The hormone production from the tumor depended neither on adrenocorticotropic hormone nor on human chorionic gonadotropin. The conclusions were that: (1) on the basis of serial measurements of serum testosterone and DHEAS, virilizing adrenal adenomas may be suspected when the concentrations of these hormones reach or exceed 200 ng/dl and 6,600 ng/ml, respectively; (2) the high-resolution CAT scanner can accurately localize these tumors; (3) cosmetic and menstrual dysfunction regressed after resection of the tumor; and (4) virilizing adrenal adenomas may produce both androgens and estrogens.