Purpose: To evaluate the feasibility of nonivasive imaging of estrogen receptors (ERs) in primary and metastatic breast cancer with the iodine- 123-labeled ER-specific ligand cis-11β-methoxy-17α-iodovinylestradiol- 17β (Z-[123I]MIVE) using conventional nuclear medicine techniques. Patients and Methods: Z-[123I]MIVE planar scintigraphy and single-photon emission computed tomography (SPECT) were performed in 12 patients with proven primary breast cancer and 13 patients with proven or from other imaging modalities evident bone, liver, lung, pleura and/or lymph node metastases. The results were compared with those of ER immunohistochemistry (IHC). Blocking studies with the antiestrogen tamoxifen were performed to test whether Z-[123I]MIVE tumor uptake was ER-mediated. Results: Planar imaging showed uptake in 11 of 12 primary carcinomas, ER IHC performed for nine of these was positive. For the planar scintigraphy-negative patient, SPECT was faintly positive, but ER IHC negative (agreement, 90%). In nine of 13 metastatic patients, planar scintigraphy was positive. The agreement between the results of ER IHC on the original primary tumor and of Z- [123I]MIVE scintigraphy was 82%. Specificity of tumor Z-[123I]MIVE uptake was established by complete blockade of uptake by tamoxifen, except in two patients who showed progressive disease. Z-[123I]MIVE scintigraphy also enabled discriminating metastases from confounding nonmalignant abnormalities of the bone scan. Conclusion: Z-[123I]MIVE scintigraphy shows high sensitivity and specificity for the detection of ER-positive breast cancer. This may have impact on diagnostic possibilities and therapeutic management. Since ER imaging shows the functional status, addressing known intratumoral and intertumoral ER heterogeneity, it may improve the characterization of disease and the selection of patients who may benefit from hormonal therapy.