This study reports on the uptake of (99m)Tc-RP527 by human breast carcinoma and its relationship to gastrin-releasing peptide receptor (GRIP-R) expression as measured by immunohistochemistry (IHC). Methods: Nine patients referred because of a clinical diagnosis suggestive of breast carcinoma and 5 patients with tamoxifen-resistant be-m breast carcinoma underwent 99mTc-RP527 scintigraphy. The findings were compared with routine staging examinations in all patients and with routine histology and IHC GRP-R staining in the first 9 patients. All 9 patients with suspected breast lesions were tumor positive. Results: The uptake of (99m)Tc-RP527 was evident in the primary tumor in 8 of 9 patients and in involved lymph nodes and part of the distant metastasis limited to the bone when present. (99m)Tc-RP527 uptake was not found in any of the tamoxifen-resistant patients. Conclusion: Uptake by primary breast carcinoma was significantly correlated with the presence of GRP-Rs as assessed by means of IHC.