Background: ctDNA-based post-surgical detection of molecular residual disease (MRD) is known to be predictive of a high risk of recurrence. Here, we report the first results of BESPOKE CRC, a multicenter, prospective, observational study evaluating the ability of a tumor-informed ctDNA assay to inform ACT treatment decisions in stage II/III CRC patients (pts). Methods: Of the 1792 pts enrolled between 2020-07-02 and 2022-08-25, plasma samples from the first 350 pts with stage II-III CRC were analyzed. ctDNA was detected and quantified using a personalized, tumor-informed assay (Signatera, Natera, Inc.). Following curative resection, 232 pts received ACT and 118 underwent observation. Results: The cohort included 154 stage II and 196 stage III CRC pts; the median follow-up was 24.8 months. ctDNA results at the post-op MRD time point (tp) were available for 295 pts; 15.6% (46/295; stage II: 9/130=6.9%; stage III: 37/165=22.4%) of pts were ctDNA positive (ctDNA+) at MRD tp (MRD+). MRD-positivity was significantly associated with inferior disease-free survival (DFS) in stages II-III combined (HR=20.8, 95% CI: 10.0-43.4, p<0.0001) and in stage-stratified subgroups (stage II: HR=25.7, 95% CI 6.8-96.7; stage III: HR=18.1, 95% CI 7.3-45.1). Within the MRD+ group, pts receiving ACT had longer DFS compared to those in the observation group (median DFS: 18.7 vs 6.7 months; HR=3.9, 95% CI: 1.3-11.5, p=0.01). In contrast, no benefit of ACT was observed in MRD- pts (HR=1.1, 95% CI: 0.3-3.9, p=0.89). Of the MRD+ pts, 39.1% (18/46) had ctDNA clearance at 12-weeks post-surgery tp. Pts with ctDNA clearance had longer DFS compared to those who remained positive (median DFS: 24.2 vs 13.8 months; HR=0.4, 95% CI 0.1-1.0, p=0.045), however, had worse DFS than pts who were ctDNA- at both 4- and 12-weeks (HR=22.5, 95% CI: 6.8-75.0, p<0.0001). Notably, 44.4% (8/18) pts with ctDNA clearance recurred; all 8 turned back ctDNA+ before radiological detection of relapse. ctDNA results during surveillance were available for 339 pts, of whom 8.3% (58/339) were ctDNA+ and had significantly worse DFS compared to serially ctDNA- pts (HR=124.3, 95% CI: 29.8-518.7, p<0.0001). Conclusions: ctDNA-based MRD detection of MRD was highly prognostic of recurrence in an early representative subset of BESPOKE CRC cohort. Data from the expanded cohort will be presented at the meeting. ctDNA MRD results were also predictive: significant benefit from ACT was observed in MRD+ but not in MRD- pts. Additionally, early ctDNA clearance in response to adjuvant therapy and ctDNA status during surveillance were prognostic of pt outcomes. Our results highlight the potential utility of ctDNA-guided adjuvant therapy in pts with stage II/III CRC. The results of BESPOKE CRC herein, as one of the first ctDNA-based prospective studies, will be further validated by ongoing ctDNA-directed randomized clinical trials. Clinical trial information: NCT04264702.