Predictors of survival in patients with stage II/III gastric cancer (GC) who received tegafur/gimeracil/oteracil (S-1) adjuvant chemotherapy (ACT) subsequent to gastrectomy were examined. Additionally, the association between dose intensity of S-1 and survival rate was investigated. A total of 62 patients with stage II/III gastric cancer were retrospectively evaluated, each of whom had received a curative D2 gastrectomy and S-1 ACT. The relative performance (RP; administered/planned S-1 doses x100%), body mass index (BMI), prognostic nutritional index (PNI) and body weight (BW) were calculated, and the association of survival with these factors and other clinicopathological parameters was examined. The 1-year treatment continuation rate for S-1 was 94.2%, excluding patients who experienced cancer recurrences during their ACT year. The initial S-1 reduction rate was 38.7%. Patients with stage II/IIIA disease exhibited significantly improved 5-year overall survival rates compared with patients with stage IIIB GC, 81.6/73.7 vs. 33.8% (P<0.01). No association between RP and survival was observed. BMI, BW and PNI were significantly decreased following surgery compared with preoperative states. In the univariate analysis, postoperative BW loss (BMI loss), pathological stage and >7 lymph node metastases were significantly associated with outcome (P<0.05); in the multivariate analysis, postoperative BW loss >10.6% and pathological stages were independent prognostic factors for survival. Continuing S-1 ACT for the full year exhibited a greater effect on survival compared with dosage. Early postoperative nutritional deterioration may decrease the survival rates in these patients.