Metastatic carcinomas of unknown origin comprise 10% to 15% of new referrals to oncology clinics. Although the cost of attempting to determine the site of origin by using immunohistochemical tests may be as high as $2000 per patient, this cost is still remarkably lower than the cost associated with the clinical evaluation. True cost effectiveness studies of the pathologic evaluation of carcinomas of unknown origin using immunohistochemistry have yet to be performed.