Background: The clinical utility of longitudinal monitoring with circulating tumor DNA (ctDNA) as a prognostic and a predictive biomarker was recently reported in patients with resectable colorectal cancer (Kotani D, et al. Nature Medicine 2023). Here we report a prospective, observational study of longitudinal ctDNA monitoring in patients with esophageal squamous cell carcinoma (ESCC) receiving neoadjuvant chemotherapy (NAC) followed by surgery. Methods: Patients with ESCC planned for NAC followed by curative surgery, were prospectively enrolled in this study. As per the protocol, plasma samples were collected at 7 timepoints: pre-NAC, post-NAC, and 4,12, 24, 36, and 48-weeks after surgery. CT scans were conducted every 3 months. Longitudinal ctDNA testing was performed using a personalized, tumor-informed ctDNA assay (Signatera, bespoke mPCR, NGS assay). In this initial report, ctDNA results at 3 time points (pre-NAC, post-NAC, and 4 weeks post-surgery) were analyzed and their association with clinical outcomes and tumor regression grade (TRG) scores were assessed. Results: So far, a total of 83 blood samples from 28 patients with ESCC were collected serially at pre-NAC (N=28), post-NAC (N=27), and post-surgical (N=28) time points. Pre-NAC ctDNA was detectable in 96.4% patients (27/28), and detection rates decreased at the post-NAC (37%; 10/27) and post-surgical (11%; 3/28) timepoints. ctDNA-positivity post-NAC and post-surgery was associated with a significantly shorter recurrence free survival (RFS) with HR of 7.0 (95% CI: 1.4-35.0; p = 0.02) and HR of 14 (95% Cl 3.0-62; p <0.001), respectively. Notably, patients who achieved ctDNA clearance with NAC had favorable RFS as compared to the patients who did not (HR=0.15, 95% CI: 0.03 – 0.75; p = 0.02). Post-NAC ctDNA was also associated with tumor regression grade (TRG) score. None of the patients with TRG3 score (N=3) were ctDNA positive, whereas 7.7% (1/13) patients in the TRG2 category and 81.8% (9/11) in the TRG1 category were ctDNA-positive. Conclusions: Our study demonstrates a strong correlation between ctDNA status and clinical outcomes in ESCC patients. Assessment of ctDNA may be useful monitoring in patients with ESCC post NAC/surgery and may be used in the future to direct post-surgical management in this patient population. Clinical trial information: UMIN000042003.