PURPOSE: Lymph node ratio (LNR) is suggested to address the shortcomings of using only lymph node yield (LNY) or status in colorectal cancer (CRC) prognosis. This study explores how LNR affects survival in metastatic colorectal cancer (mCRC) patients. METHODS: This observational cohort study investigated Stage IV CRC patients (1995-2021) who underwent an upfront resection of their primary tumour at Concord Hospital, Sydney. Clinico-pathological data were extracted from a prospective database, and LNR was calculated both continuously and dichotomously (LNR=0 and LNR>0). The primary endpoint was overall survival (OS). The associations between LNR and various clinico-pathological variables were tested using regression analyses. Kaplan–Meier and Cox regression analyses estimated OS in univariate and multivariate survival models. RESULTS: 464 patients who underwent a primary CRC resection with clear margins (mean age 68.1 years [SD 13.4]; 58.0% M; colon cancer [n=339,73.1%]) had AJCC Stage IV disease. The median LNR was 0.18 (IQR:0.05-0.42) for colon cancer (CC) resections, and 0.21 (IQR:0.09-0.47) for rectal cancer (RC) resections. 84 patients had an LNR=0 (CC=66 patients; RC=18 patients). The 5-year OS for the CC cohort was 10.5% and 11.5% for RC. Increasing LNR was associated with poorer OS in both CC (P=0.02) and RC (P<0.001). Patients with non-lymphatic dissemination only (LNR=0) had better survival compared to those with lymphatic spread (i.e. LNR>0) ([CC:P=0.02, RC:P=0.02]). CONCLUSION: LNR is worthy of consideration in patients with mCRC. LNR=0 patients have a better prognosis, underscoring the need for adequate lymphadenectomy to facilitate precise mCRC staging.