Background: Fertility is important to many young breast cancer survivors (YBCS), who face difficult decisions on whether to undergo fertility preservation prior to treatment. Because few longitudinal data assessing decisional regret are available, the objectives of this study were to assess longitudinal changes in decisional regret on fertility preservation following breast cancer diagnosis; determine if fertility preservation treatment decisions are related to decreased decisional regret. Methods: From 3 academic breast cancer programs, 169 YBCS younger than age 45 were recruited at diagnosis between 2009 and 2012 and followed prospectively for ovarian function. Participants completed questionnaires on fertility preservation choices and the Decisional Regret Scale (DRS) during study visits every 6 months for up to 5 years. DRS is scored 0 (no regret) to 100 (highest regret). DRS was dichotomized as none versus any decisional regret. Generalized linear models estimated the change in DRS over time and the association between patient characteristics and DRS. Results: Mean age at diagnosis was 38.7 (SD 4.8). Median total follow-up was 176 days (IQR 84 to 1415 days). Enrollment DRS was available for 89 women; 48% reported decisional regret about fertility preservation (median DRS=20). Participants worried about future fertility were more likely to report decisional regret (p=0.009). 31% underwent fertility preservation, but this was not associated with decisional regret (p=0.65). In repeated measures analysis for the entire cohort, no significant change in DRS occurred over this time period (OR 0.8, 95% CI 0.4-1.7). Worry about future fertility remained significantly associated with DRS over time (OR 55.1, 95% CI 7.7-395.1). Conclusions: In a cohort of YBCS, experiencing decisional regret about fertility preservation persists for years after diagnosis. Those worried about future fertility are more likely to experience decisional regret regarding fertility preservation.