Abstract Introduction Addition of ovarian function suppression to conventional endocrine therapy alone in premenopausal women provides a survival benefit with moderate to high risk hormone-receptor positive breast cancer, especially who received chemotherapy Prediction of menstruation recovery after chemotherapy is important for deciding subsequent endocrine treatment and addressing fertility issues. Methods In the adding OFS after chemotherapy trial (ASTRRA), patients who resumed ovarian function up to 2 years after chemotherapy were randomized to receive either 5 years of tamoxifen or adding 2 years of OFS with tamoxifen. With these 1383 patients, we developed a model that predicts when patients recover menstruation within 3 years after chemotherapy using several variables including age, BMI, chemotherapy regimen and duration, serum E2 and FSH level. Results A total of 1017 patients data were used to develop prediction model and 366 patients data were used for external validation. In development group, 546 (53.6%) patients resumed menstruation during follow up period of 5 years. In multivariable analysis, younger age and AC based regimen without taxane were strong predictive factor for menstruation recovery. However predictive value of chemotherapy regimen was not constant over time. Therefore, we conducted another model with patients (n= 624) who did not recover menstruation within one year. In this patient group, predictive factors for menstruation recovery was age and serum E2 level at 6 months after chemotherapy. We also conducted a simplified scoring system to estimate change of recovery by using risk factors mentioned above. Conclusion Younger age is an important persisting factor predicting menstrual recovery after chemotherapy. Although chemotherapy regimen predicts shor-term menstrual recovery, over time, patient factors have more predictive influence on recovery. Recovery of serum E2 level would be important to predict subsequent menstruation recovery. Citation Format: Young Joo Lee, Woo C Noh, Seok J Nam, Byeong-Woo Park, Eun S Lee, Seock A Im, Yong S Jung, Jung H Yoon, Sung S Kang, Kyong H Park, Soo-Jung Lee, Min H Lee, Joon Jeong, Sung Y Kim, Hyun-Ah Kim, Se-Hwan Han, Wonshik Han, Min H Hur, Seonok Kim, Sei-hyun Ahn, Hee J Kim. Prediction of menstruation recovery timing in premenopausal breast cancer patients taking tamoxifen after chemotherapy: An ASTRRA substudy [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P2-10-07.