The prognostic role of the preoperative lymphocyte-to-monocyte ratio (LMR) in patients with gastric adenocarcinoma (GC) remains unclear. The aim of this study was to explore the prognostic potential of the preoperative LMR in patients with resectable GC.The medical records of 926 consecutive patients with resectable GC between 2005 and 2010 were retrospectively reviewed and analyzed. Patients were stratified into two groups based on the preoperative LMR with a cutoff value of 4.8 (group 1: LMR ≤ 4.8; group 2: LMR > 4.8). Clinicopathologic factors potentially affecting patient outcomes were collected prospectively and analyzed.Compared to group 2, in group 1, there was a higher percentage of men, patients aged >48 y, total gastrectomy, tumor size > 4.8 cm, T4 lesions, N3 disease, metastatic tumors, advanced stage, ratio of metastatic to examined lymph nodes > 0.18, R1 resection, and occurrence of vascular or lymphatic invasion. Group 1 also had a higher 30-d surgical mortality rate (groups 1 versus 2 at 2.9% versus 0.5%; P = 0.006) and lower 3-y and 5-y overall survival (53.6% versus 71.9% and 46.4% versus 63.3%, respectively; P < 0.0001). Multivariate analysis showed that preoperative low LMR was an unfavorable prognostic factor for resectable GC.Patients with lower LMR had more aggressive tumor behavior, higher surgical mortality rates, and worse long-term survival. The preoperative LMR may serve as an independent prognostic factor for prediction of surgical outcomes and for assisting clinicians in determining future treatment plans.