Background: In-vitro and in-vivo evidence suggests that aspirin may have an anti-tumor effect. Multiple epidemiologic studies have reported improved breast cancer survival among regular aspirin users compared to non-users. Pooled data from randomized trials of aspirin for cardiovascular disease have also reported a decreased risk of metastatic cancer among aspirin users. Thus, we conducted a prospective randomized controlled trial to determine the true benefits and risks of adjuvant aspirin therapy for breast cancer survivors. Methods: The primary objective was to compare the effect of 300 mg aspirin daily versus placebo upon invasive disease-free survival (iDFS) in patients with high-risk, HER2-negative breast cancer. Secondary objectives included effects on overall survival, cardiovascular disease, toxicity, and adherence. Eligible participants included patients aged 18-70 diagnosed with a primary invasive HER2-negative breast cancer. If hormone receptor (HR)–positive, tumors needed to be node positive and diagnosed within the past 10 years. If HR negative, tumors could be node positive or T2-4N0 and diagnosed within the past 18 months. Participants were randomly selected (1:1) to aspirin 300 mg versus placebo daily for 5 years in a double-blinded fashion. Stratification factors include HR status (positive vs. negative), body mass index (< or ≥ 30 kg/m2), and stage (II vs. III). Based upon an accrual goal of 2,936 patients to reach 381 iDFS events, the study was estimated to have 80% power to detect HR 0.75. Results: From January 2017 to December 2020, 3,021 participants were enrolled. Treatment arms were well balanced in terms of key characteristics. In November 2021, the Data Safety and Monitoring Board recommended that the trial be unblinded because the stratified hazard ratio had crossed a pre-specified futility boundary. After 191 iDFS events (aspirin: 107, placebo: 84) and median follow-up of 20 months, the stratified hazard ratio comparing aspirin to placebo was 1.27 (z-score: -1.64), which is greater than the pre-specified hazard ratio of futility 1.03 (z-score < -0.192). There was no difference in the frequency of grade 3/4 adverse events by study arm. Compliance was high and similar across arms. Non-protocol use of aspirin/non-steroidal anti-inflammatory drugs was similar across arms and less than 14%, consistent with prior randomized aspirin trials. Updated results on iDFS events will be provided at presentation. Conclusions: In this double-blinded, placebo-controlled, randomized trial, there was no benefit in breast cancer invasive disease-free survival with the addition of 300 mg aspirin daily. Although inflammation may still play a role in cancer progression, aspirin is not recommended for prevention of breast cancer recurrence. Clinical trial information: NCT02927249.