Thirteen patients with primary breast masses were studied with positron emission tomography (PET) and 16 alpha-[fluorine-18]-fluoroestradiol-17 beta. PET images demonstrated uptake of the labeled estrogen analog at sites of primary carcinomas and in several foci of axillary lymph node metastases, as well as in one distant metastatic site. There was excellent correlation between uptake within the primary tumor measured on the PET images and the tumor estrogen-receptor concentration measured in vitro after excision (r = .96). This technique may provide an in vivo method of assessing estrogen receptors in primary and metastatic breast cancers and thus guide management of this disease with antiestrogen chemotherapy.