The aim of this study was to examine the effect of being overweight on survival in patients with gastric cancer undergoing adjuvant chemoradiotherapy and chemotherapy. In this study 152 patients were evaluated. Radiotherapy dose was 45 Gy given in 5 weeks. 5-FU 425 mg/m(2) and folinic acid 20 mg/m(2) were administered weekly during the radiotherapy and four cycles with 4-week intervals as consolidation chemotherapy after radiotherapy. Patients were assigned into two groups according to their body mass index: overweight (body mass index >= 5 kg/m(2)) and normal weight (body mass index <25.0 kg/m(2)). The median overall survival was 39 months vs. 18 months and median disease-free survival was 27 months vs. 13 months in the overweight and normal-weight groups respectively (P = 0.004 and P = 0.006 respectively). The 5-year survival was better in the patients with overweight than those with normal weight (42% vs. 17%; P = 0.004). The overall survival was significantly better with being overweight and early pathological stage (P = 0.016 and P = 0001 respectively). Overall survival, disease-free survival and long-term survival in patients with gastric cancer undergoing adjuvant treatment were better in overweight than normal-weight patients. Moreover, it was shown that body mass index and pathological stage were associated to survival and prognosis.