Background: The optimal approach to adjuvant chemotherapy for rectal cancer is keenly debated. Routine practice and clinical guidelines vary widely. After pre-operative chemoradiation (CRT), a pathologic complete response (pCR) or nodal involvement (pN+) are prognostic markers that can guide clinical decision-making, but markers that better define the patients (pts) that are likely or unlikely to benefit from chemotherapy are urgently needed. We investigated the potential role of ctDNA as a biomarker to guide therapy. Methods: We conducted a prospective, multi-centre study in pts with LARC (T3/T4 and/or N+) planned for CRT and curative resection. Serial plasma samples were collected pre-CRT, post-CRT, and 4-10 weeks after surgery. Somatic mutations in individual pts’ tumor were identified via sequencing of 15 genes commonly mutated in colorectal cancers. We then designed personalized assays to quantify ctDNA in plasma samples. Pts received adjuvant therapy at clinician discretion. Results: 200 pts were enrolled between Apr-2012 and Dec-2015. Median age was 62 years (range 28-86), 67% were male and 159 pts had pre-CRT and post-op ctDNA samples available for analysis. Of these, 122 (77%) pts had detectable ctDNA prior to therapy. After surgery, 19 pts had detectable ctDNA and 11 of these 19 (58%) have recurred during a median follow up of 22 months. Recurrence occurred in only 12 of 140 (8.6%) with negative ctDNA (HR 12, p < 0.001). One hundred and two (64%) pts received adjuvant chemotherapy. Post-op ctDNA detection was predictive of recurrence irrespective of adjuvant chemotherapy (chemo: HR 10, p < 0.001; no chemo: HR 16, p < 0.001). Thirty-four pts (21%) achieved a pCR, 43 (27%) had pN+ disease. pCR (vs non-pCR) was associated with a trend for lower recurrence risk (HR 0.31, p = 0.089) and pN+ (vs pN0) with a higher recurrence risk (HR 4.2, p < 0.001). ctDNA detection remained predictive of recurrence among pts with a pCR (HR 14, p = 0.014) or with pN+ disease (HR 11, p < 0.001). Conclusions: Post-op ctDNA analysis stratifies pts with LARC into very high and low risk groups. ctDNA analysis remains strongly predictive of recurrence among pts with both lower risk (pCR) and higher risk (pN+) disease.