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Abstract Background 18F-FDG PET-CT is commonly used to monitor treatment response in patients with metastatic colorectal cancer (mCRC). With improvement in systemic therapy, complete metabolic response (CMR) is increasingly encountered but its clinical significance is undefined. The study examined the long-term outcomes and recurrence patterns in these patients. Methods Consecutive patients with mCRC who achieved CMR on PET-CT during first-line systemic therapy were retrospectively analysed. Measurable and non-measurable lesions identified on baseline PET-CT were compared with Response Criteria in Solid Tumors (RECIST) on CT on a per-lesion basis. Progression free (PFS) and Overall Survival (OS) were compared with clinical parameters and treatment characteristics on a per-patient basis. Results Between 2008 and 2011, 40 patients with 192 serial PET-CT scans were eligible for analysis involving 44 measurable and 38 non-measurable lesions in 59 metastatic sites. On a per-lesion basis, 46% also achieved Complete Response (CR) on RECIST criteria and sustained CMR was more frequent in these lesions (OR 1.727, p = 0.0031). Progressive metabolic disease (PMD) was seen in 12% of lesions, with liver metastasis the most common. Receiver operating characteristics (ROC) curve analysis revealed the optimal value of SUVmax for predicting PMD of a lesion was 4.4 (AUC 0.734, p = 0.004). On a per-patient basis, 14 patients achieved sustained CMR and their outcomes were better than those with PMD (median OS not reached vs 37.7 months p = 0.0001). No statistical difference was seen in OS between patients who achieved PR or CR (median OS 51.4 vs 44.2 months p = 0.766). Conclusion Our results provided additional information of long-term outcomes and recurrence patterns of patients with mCRC after achieving CMR. They had improved survival and sustained CMR using systemic therapy alone is possible. Discordance between morphological and metabolic response was consistent with reported literature but in the presence of CMR the two groups had comparable outcomes.