Background: Radiographic surveillance of colorectal cancer (CRC) after therapy is costly and insensitive. We have reported high cross-sectional performance of methylated DNA markers (MDMs) for detection of primary CRC and metastatic recurrence. This study evaluated MDMs to detect recurrence and measure response to anti-cancer therapy in a longitudinal cohort. Methods: This was a nested case-control study drawn from a prospective cohort of adult patients with CRC who completed definitive treatment and followed forward from the start of surveillance by cross-sectional imaging (CT or MRI) and carcinoembryonic antigen (CEA), collected every 3-6 months under routine care. Blood was collected in LBgard® tubes (Biomatrica, San Diego CA). Cases had recurrent CRC (defined by RECIST v1.1 or radiographic findings that justify re-initiation of anti-cancer therapy as determined by treating clinicians). Controls remained NED (no radiographic recurrence and off therapy for ≥4 surveillance visits). From 6mL of plasma, MDMs (CNNM1, ANKRD13B, FER1L4, ZNF568, CHST2, ZNF671, VAV3, QKI, GRIN2D, DTX1, PDGFD, SFMBT2, JAM3) were measured by target enrichment long-probe quantitative-amplified signal assays, normalized to B3GALT6, in blinded fashion. Previously published random forest models of MDMs +/- CEA, developed on independent patients with intact primary tumors, were used to develop a summary score. Results: With a median follow-up of 276 days (IQR 187-343) and 3 visits per patient (54 visits total), 18 patients had recurrence after start of surveillance. Another 18 patients remained NED after median 555 days (IRQ 469-662) and 4 surveillance visits per patient (76 visits total). Using a 90^th %-ile cut-off summary score from 60 NED patients in a prior cross-sectional measurement, the MDM panel was positive in 15 of 18 patients who recurred and only 2 of the 18 NED patients (Table). Of the 76 patient visits for NED patients, 67 (88%) were below the threshold. The MDM panel score crossed the positivity threshold by a median of 127 days (IRQ 0-233) preceding clinical or radiographic documentation of recurrence. A random forest model with CEA was more sensitive but less specific (Table). In patients who resumed therapy, the MDM panel scores correlated closely with subsequent RECIST scores (not shown). Conclusions: Plasma MDMs rise in anticipation of recurrent CRC prior to radiographic detection and remain low in patients without radiographic disease over serial measurements, warranting further development of this tumor-agnostic liquid biopsy approach as a cost-effective tool to assist cancer surveillance.