Aim: The lymphocyte-to-monocyte ratio (LMR) is useful for predicting the prognosis of patients with gastric cancer (GC) and those with colorectal cancer (CRC) undergoing surgery. The relationship between the LMR and postoperative outcome of patients with early-stage gastrointestinal cancers such as stage I GC and CRC remains unclear. Methods: We retrospectively evaluated 323 stage I GC and 152 stage I CRC patients undergoing surgery. Univariate and multivariate analyses using the Cox proportional hazards model were performed to identify the clinical characteristics associated with overall survival (OS), and the cutoff values of these variables were determined by receiver operating characteristic analysis. The Kaplan-Meier method and log-rank test were used for postoperative survival comparisons according to the LMR (GC: LMR < 4.2 vs ≥4.2; CRC: LMR < 3.0 vs ≥3.0). Results: Univariate and multivariate analyses revealed that OS was significantly associated with the LMR (<4.2/≥4.2) (HR, 2.489; 95% CI, 1.317-4.702; P = 0.005), as well as age (>75/≤75 years) (HR, 3.511; 95% CI, 1.881-6.551; P < 0.001) and albumin level (≤3.5/>3.5 g/dL) (HR, 3.040; 95% CI, 1.575-5.869; P = 0.001), in stage I GC patients. Survival analysis demonstrated a significantly poorer OS in stage I GC patients with a LMR < 4.2 compared with ≥4.2 (P < 0.001). In stage I CRC patients, despite a significant difference in OS according to the LMR (<3.0 vs ≥3.0) (P = 0.040), univariate analysis revealed no significant association between the LMR and OS. Conclusion: LMR is a useful predictor of the postoperative outcome of stage I GC patients treated surgically. K E Y W O R D S immunosuppression, lymphocyte-to-monocyte ratio, stage I colorectal cancer, stage I gastric cancer This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.