Background: The DATA study compared 6 and 3 years of anastrozole therapy in postmenopausal women with hormone-receptor positive early breast cancer previously treated with 2-3 years tamoxifen (oral presentation SABCS 2016 #S01-03). The study included postmenopausal women, allowing those with chemotherapy-induced ovarian function failure (CIOFF). However, these may be at risk of ovarian function recovery (OFR). The current analysis compared the survival of women with CIOFF with definite postmenopausal women and examined the influence of OFR on survival. Methods: We selected patients from the DATA study aged 45-57 years at randomization who had received (neo-)adjuvant chemotherapy. They were classified by menopausal status at randomization (definite postmenopausal before chemotherapy or by ovariectomy, versus CIOFF). The latter were monitored by estradiol measurements for OFR during anastrozole. Endpoints: Disease-free Survival (DFS), Distant Recurrence-free Survival (DRFS), and Overall Survival (OS), corrected for tumor size, nodal status, grade, and hormone-receptor status. We used the landmark method to calculate residual 5-year survival rates. Results: In total, 261 patients were definite postmenopausal and 395 had CIOFF, of whom 39 experienced OFR while 290 did not (66 were excluded from the landmark analysis because follow-up estradiol levels were lacking). When comparing the CIOFF with the definite postmenopausal women, the 5-year survival rates were not significantly different. Within the group with CIOFF, experiencing OFR was associated with a trend for worse outcome (DFS-event HR 1.33 (95% CI 0.61-2.90), P=0.48; DRFS-event HR 2.11 (95% CI 0.89-5.02), P=0.09; and OS-event HR 2.24 (95% CI 0.92-5.45), P=0.07). Patients who experienced OFR in the first year had a residual 5-year rate for DFS of 73.1% compared with 87.4% in those who did not. For DRFS these rates were 76.9% vs. 92.1%, and for OS 80.8% vs. 94.4% respectively. Conclusions: These results suggest that women with CIOFF undergoing anastrozole treatment may be at increased risk of disease recurrence if experiencing OFR despite close monitoring of estradiol levels and adjusting endocrine treatment. Clinical trial information: NCT00301457