摘要：Abstract Background The relationship between preoperative body mass index (BMI) and the survival of postoperative gastric cancer patients is not clear. Furthermore, the survival impact with postoperative BMI is not known, even though weight loss is inevitable after gastrectomy. Methods Patients who underwent gastrectomy for gastric cancer between 2000 and 2008 were included in the study (n = 1909). Patients were divided into three groups based on their BMIs: low (<18.5 kg/m2), normal (18.5–24.9 kg/m2), and high BMI (≥25.0 kg/m2). Patient survival was compared according to BMI at two time points: baseline and 1 year after surgery. Results Regarding BMI 1 year after surgery, overall survival, disease-specific survival, and recurrence-free survival were longer in the high BMI group than the low and normal BMI groups. In a Cox proportional hazards model, adjusting for the patient's age, sex, type of surgery, tumour stage, histology, curative resection, and BMI at baseline, a high BMI 1 year after surgery was associated with lower overall mortality compared to normal BMI (hazard ratio 0.51; 95% confidence interval, 0.26–0.98). However, BMI at baseline was not an independent prognostic factor. Conclusion BMI 1 year after surgery significantly predicted the long-term survival of patients with gastric cancer compared with the preoperative BMI.