651 Background: 25% of invasive breast cancers occur in premenopausal women. Adjuvant chemotherapy may induce amenorrhea, resulting in infertility, influencing hormonal therapy options, and potentially increasing risks of late toxicity. Methods: We retrospectively surveyed women who were premenopausal and < age 50 at initiation of chemotherapy to determine if giving T after AC (AC → T) altered rates of chemotherapy induced amenorrhea (CIA). Women who received either AC x 4 or AC x 4 followed by 3 months of T (either paclitaxel or docetaxel) with or without adjuvant tamoxifen were recruited from a dedicated breast clinic and the Young Survival Coalition. CIA was defined as cessation of menses within one year of starting chemotherapy and lasting ≥ 6 months. Results: 195 eligible patients completed the survey. 160 were ≤ 40. 35 were > 40 but < 50. 77 patients received AC, 118 received AC → T. In patients ≤ 40, the rate of CIA was greater for those who received AC → T (61%) as compared to AC (44%), p=0.04 (OR 1.97, 95% CI 1.01 to 3.84). Many of these young women subsequently resumed menstruating (33% receiving AC, 43% receiving AC → T). Women > 40 had a high likelihood of experiencing CIA with both AC (81%) and AC → T (84%), p=0.35 (95% CI 0.20 to 8.31) and for 82% this was permanent. Results of multivariate logistic regression analysis show that the odds of developing CIA was nearly 5 fold higher for women > 40 than for those ≤ 40 (p=0.002). Those receiving AC → T had an odds ratio of CIA of 1.9 compared to those receiving AC (p=0.05). Tamoxifen did not influence CIA in either group of patients. Conclusion: The addition of T to AC increases the risk of CIA in patients ≤ 40. Women ≤ 40 who experience CIA have a substantial chance of resuming menstruation even after 6 months of amenorrhea and this should be carefully considered when planning hormonal therapy and contraception in this age group. No significant financial relationships to disclose.