Background: Residual disease after treatment for colorectal cancer (CRC) poses a risk for recurrence but imaging and CEA are limited in their capacity to detect residual disease. A simple test is needed for assessing treatment response. This study determined whether levels of methylated BCAT1/IKZF1 DNA in blood correlate with tumor burden and whether post-treatment levels inform efficacy of different treatments for CRC. Methods: Patients with primary CRC had blood collected prior to treatment (n = 282, 59.9% males, median age 68.5y). Cell free DNA (cfDNA) was extracted from plasma and assayed for methylation in BCAT1 and IKZF1. Detection of methylation in either gene deemed a sample positive; levels were expressed as %methylation (average methylation/average cfDNA). Positive patients had additional samples collected post-treatment for early stage CRC (surgery, n = 31), advanced/metastatic CRC (surgery + adjuvant chemotherapy, n = 15), and rectal cancer (neoadjuvant therapy, surgery +/- chemotherapy, n = 6), or following mid-therapy suspension of treatment in advanced CRC (n = 24). Tumor size was expressed as the maximum diameter of the primary (assessed surgically or by MRI). Results: Pretreatment results increased with CRC stage. Positivity by stage was: I, 23.7% (14/59); II, 62.1% (54/87); III, 68.6% (70/102); IV, 85.3% (29/34). Level by stage: I, 0.0%; II, 0.06%; III, 0.07%; IV, 4.07%, p < 0.001). Pretreatment levels correlated significantly with tumor size (r = 0.372, p < 0.001). Post-treatment blood was collected a median 2.4mo (IQR 1.7-3.9) after therapy completion. Positivity decreased after completing treatment (Table), with 88.4% of cases (46/52) becoming ctDNA negative. All cases with complete treatment had a reduction in biomarker levels, whereas in those with incomplete therapy, 54.5% (12/22) remained positive and the pre- and post-treatment levels were not significantly different. Of those positive after treatment, 13 had a further blood sample: 8 had become ctDNA negative and all but one remained disease free. Five remained positive and all had further suspected or confirmed disease. Conclusions: Levels of methylated BCAT1 and IKZF1 DNA in blood correlated with tumor burden; levels became undetectable in the majority of patients following completion of planned curative intent treatment. The methylated ctDNA blood test aids monitoring of responses to therapy and identification of those cases with residual cancer who might benefit from ongoing therapy.