Background: Adjuvant CT in stage III CC prevents recurrence by eradicating minimal residual disease (MRD) not visible on imaging. However, many patients (pts) will not have MRD and not all pts with MRD will benefit from standard CT. In this study, we determined (i) if the presence of ctDNA following surgery was predictive of recurrence following CT; (ii) if ctDNA could be used to determine the effectiveness of CT during treatment and (iii) if the presence of ctDNA following CT completion was predictive for later recurrence. Methods: Serial plasma samples from stage III CC pts planned for adjuvant CT were collected post-surgery, during CT and at treatment completion. Somatic mutations in individual tumors were identified via massively parallel sequencing of 15 genes commonly mutated in colorectal cancer. Personalized Safe-SeqS assays to quantify ctDNA in plasma samples were designed. Clinicians were blinded to ctDNA results. Results: 95 pts were enrolled from Nov-2014 to May-2017, median age was 64 years. All received adjuvant CT and 19 (20%) had recurred at a median follow-up of 21.1 months. We observed an inferior recurrence-free survival (RFS) in the 19 of 95 pts (20%) with positive ctDNA post-surgery (HR, 3.52; p = 0.004). ctDNA status changed from positive to negative in 10 of 17 pts (59%) after 2 months of CT; and 9 of 18 pts (50%) at CT completion. Superior RFS was observed when ctDNA became undetectable after CT (HR 5.11; p = 0.02). Conversely, ctDNA status changed from negative to positive after CT in 6 of 71 pts (8%) and was associated with an inferior RFS (HR 5.30; p = 0.006). Finally, inferior RFS was seen in the 15 of 89 (17%) with positive ctDNA after adjuvant CT completion (HR, 7.14; p < 0.001). Conclusions: ctDNA can reveal the presence of residual metastatic cancer cells not apparent on imaging in stage III CC patients. Serial analysis of ctDNA can define subsets of pts benefiting or not benefiting from CT and is therefore a real-time marker of adjuvant treatment efficacy in solid tumors. Further studies are needed to define how best to use ctDNA analysis to guide a personalized and risk adjusted approach to the initiation and modification of adjuvant CT in stage III CC.