PURPOSE: This study aimed to evaluate the association between low body mass index (BMI) and morbidity after gastric cancer surgery. METHODS: A total of 1,805 patients were included in the study. These subjects had undergone gastric cancer surgery at a single institution between January 1997 and December 2013. Clinicopathologic and morbidity data were analyzed by dividing the patients into 2 groups: underweight patients (BMI < 18.5 kg/m2) and nonunderweight patients (BMI ≥ 18.5 kg/m2). RESULTS: The overall complication rate as determined by our study was 24.4%. Pulmonary complications occurred more frequently in the underweight group (UWG) than in the non-UWG (10.5% vs. 3.8%, respectively; P = 0.012). Multivariate analysis revealed two independent factors responsible for postoperative pulmonary complications—weight of the patients (UWG vs. non-UWG, 10.8% vs. 3.8%; P < 0.007) and stage of gastric cancer (early stage vs. advanced stage, 3.1% vs. 6.8%; P < 0.023). Multivariate analysis revealed that underweight (UWG vs. non-UWG, 10.8% vs. 3.8%, respectively, P < 0.007) and advanced cancer stage (early stage vs. advanced stage, 3.1% vs. 6.8%, respectively, P = 0.023) were significant risk factors for postoperative pulmonary complications. CONCLUSION: We concluded that underweight patients had a higher pulmonary complication rate. Additionally, underweight and advanced cancer stage were determined to be independent risk factors for the development of postoperative pulmonary complications.