Abstract: Objective To explore the relationship between FAS expression in CTCs and estradiol therapy in endocrine-resistant advanced breast cancer and the effect of estradiol in reversing endocrine-resistance.Methods 20 endocrine-resistant advanced breast cancer patients with FAS expression in CTCs were given estradiol 2mg,three times a day untill the disease advanced.Then,anastrozole 1 mg/d was used.Iconography and CTCs examination were implementation in every circle.Results Among 20 cases,1 case dropped out.According to RECIST1.0:CR 0,PR 2 cases(10.5%),SD 5 cases(26.3%),PD 12 cases(63.2%).ORR was 10.5%;clinical benefit was 36.8%;mPFS was 6.0 months.According to CTCs:CR 0,PR 2 cases(10.5%),SD 4 cases(21.1%),PD13 cases(68.4%).ORR was 10.5%;clinical benefit was 31.6%;mPFS was 5.0 months.After the disease advanced,anastrozole was used.According to CTCs:CR 0,PR 1(14.3%),SD 2(28.6%),PD 4(57.1%).ORR was 14.3%;clinical benefit was 42.9%;mPFS was 4.0 months.According to CTCs:CR 0,PR 1(14.3%),SD 1(14.3%),PD 5(71.4%);ORR was 14.3%;clinical benefit was 28.6%;mPFS was 3.0 months.There was no significant difference between the two exam methods,but CTCs can be detected earlier than iconography change.Conclusion Estradiol can provide clinical benefit for endocrine-resistant advanced breast cancer patient who has FAS expression in CTCs.Estradiol can reverse the aromatase inhibitor resistant in some cases with no serious adverse events.