An estrogen-receptor-binding radiopharmaceutical, 16 alpha-[77Br]bromoestradiol-17 beta, has been used successfully at high specific activity to image carcinogen-induced mammary tumors in rats and in preliminary studies to image breast tumors in patients. The biodistribution of the labeled estrogen in rats and its clearance in a monkey were used to estimate the radiation absorbed doses to a human resulting from administration of the radiopharmaceutical. Preliminary imaging studies in patients with mammary carcinoma show promising results and warrant further development of radiolabeled estrogens, particularly those carrying positron emitters that could permit positron emission tomography--for example Br-75 or F-18.