The sodium-iodide symporter (NIS), which transports iodine into the cell, is expressed in thyroid tissue and was recently found to be expressed in approximately 80% of human breast cancers but not in healthy breast tissue. These findings raised the possibility that therapeutics targeting uptake by NIS may be used for breast cancer treatment. To increase the efficacy of such therapy it would be ideal to identify a radioactive therapy with enhanced local emission. The feasibility of using the powerful beta-emitting radiometal (188)Re in the form of (188)Re-perrhenate was therefore compared with 131I for treatment of NIS-expressing mammary tumors. In the current studies, using a xenografted breast cancer model induced by the ErbB2 oncogene in nude mice, (188)Re-perrhenate exhibited NIS-dependent uptake into the mammary tumor. Dosimetry calculations in the mammary tumor demonstrate that (188)Re-perrhenate is able to deliver a dose 4.5 times higher than (131)I suggesting it may provide enhanced therapeutic efficacy.