Background: Detection of circulating tumor DNA (ctDNA) has broad clinical utility including disease monitoring, prognostication and response to chemotherapy. ctDNA is commonly detected by targeting tumor-specific features including mutations, insertions, deletions or hypermethylation. The two genes, BCAT1 and IKZF1, are methylated with high frequency in colorectal cancer (CRC). This study aimed to analyze the impact of tumor resection on ctDNA levels by assaying pre- and postoperative blood samples for methylated BCAT1 and IKZF1. Methods: 91 people (age 32-86 years, 53% male) with invasive CRC, but without neoadjuvant therapy, had blood collected prior to surgery and within 12 months (1-12 months) after resection. Cancers were clinicopathologically staged. DNA extracted from plasma was assayed for methylated BCAT1/IKZF1 and detection of either marker was deemed positive for ctDNA. Results: 47 (52%) of the 91 CRC patients were ctDNA positive before resection, including 5/30 (17%) stage I, 17/28 (61%) stage II, 23/31 (74%) stage III and 2/2 (100%) stage IV. After resection 75% (35/47) became ctDNA negative (median 2 months after resection), and all had apparent tumour clearance. Of the 35 postoperative ctDNA negative cases 22 had further surveillance CT scans within study timeframe. 86% (20/22) showed no recurrent CRC but 2 of these developed a new cancer (metachronous CRC, prostate). The remaining 2 tested ctDNA positive 14 and 25 months later and recurrence was confirmed. Of the 12 postoperative ctDNA positive cases, two were found to not have complete tumour clearance at surgery (residual disease). Follow-up CT scans were available for a further 8 patients which revealed that 4 later presented with cancer (3 recurrence, 1 thyroid) at a median 15 months after resection. The remaining 4 postoperative positive cases were negative at subsequent blood testing (median 8 months later). Conclusions: Methylated BCAT1/IKZF1 DNA in preoperative blood is dependent on tumor stage, but informs the completeness of resection given the high rate (74.5%) of ctDNA disappearance post-surgery. If cases persistently test ctDNA positive after surgery or become ctDNA positive later, residual or recurrent disease should be suspected, respectively.