Objectives: Due to the heterogeneous receptor distribution and changes of receptor status over time, the biochemical measurement of estrogen receptor status of biopsy specimens is not sufficient to diagnose breast cancer. As a result, I-123 labeled estradiols have been applied for the diagnosis. The purpose of this study was to develop a suitable radioligand for imaging estrogen receptor-positive human breast tumors. Methods: Among the various estradiol derivatives, 17α-[123I]iodovinyl estradiol ([123I]IVE) has been prepared from 17α-ethynyl estradiol. Labeling of E-17α-[123I]iodovinyl estradiol (E-[123I]IVE) was carried out using peracetic acid with [123I]NaI and Z-[123I]IVE labelling was archived using chloamine- T/HCl solution with [123I]NaI. Labeling yield was determined by silica thin-layer chromatography (TLC) and radiochemical purity was measured by high performance liquid chromatography (HPLC). The biodistribution of E-[123I]IVE was measured in immature female rats at 60 min, 120 min and 300 min after injection. Results: The labeling yield of two isomers was 92% and 94% (E-[123I]IVE and Z-[123I]IVE, respectively). The radiochemical purity was more than 98% after purification. The highest uptake was observed at 120 min in uterus (3.11% ID/g for E-[123I]IVE). Conclusion: These results suggest the possibility of using E-[123I]IVE as an imaging agent for the evaluation of the presence of estrogen receptor in patients with breast cancer.