A correlation between the lymphocyte-to-monocyte ratio (LMR) and the survival of patients with hematological malignancies has been reported previously. However, there have been few studies investigating the prognostic significance of LMR in patients with solid tumors. The aim of the present study was to evaluate the prognostic significance of preoperative LMR in patients with colorectal cancer (CRC). A total of 189 patients undergoing potentially curative surgery for CRC were enrolled. The LMR was calculated from preoperative blood samples by dividing absolute lymphocyte count by absolute monocyte count. A cut-off value of 4.8 was set based on the receiver operating characteristic curve; 116 patients were classified as high-LMR, and 73 patients classified as low-LMR. The high-LMR group exhibited significantly better relapse-free survival (P=0.0018) and overall survival (P=0.0127) rates than the low-LMR group. According to the multivariate analysis of survival, preoperative LMR was identified as an independent prognostic factor for relapse-free survival (P=0.041) and overall survival (P=0.031). Therefore, preoperative LMR is a useful prognostic marker in patients with CRC.