98 Background: The prognosis of patients with linitis plastica (type 4) and large ulcero-invasive-type (type 3) gastric cancer is reported to be extremely poor. In stage II/III gastric cancer, adjuvant chemotherapy with S-1 is a standard treatment in Japan. However, the efficacy of postoperative chemotherapy with S-1 in these types of patients with dismal prognosis is unknown. The aim of this study is to evaluate the impact of adjuvant chemotherapy with S-1 on survival in type 4 and large type 3 gastric cancer patients. Methods: A total of 152 patients with clinically resectable type 4 and large type 3 gastric cancer who underwent R0 or R1 surgery from 2002 to 2014 were included. The survival outcome between patents with surgery alone and patients who received adjuvant S-1 was compared using a 1:1 propensity score matching method. Results: Patients with adjuvant S-1 were significantly younger (67 vs 74 y, p = 0.009), had higher incidence of T4 (90 vs 62%, p < 0.001), N2-3 (84 vs 63%, p = 0.008), and cytology positive (52 vs 29%, p = 0.006) than in surgery alone patients. Before matching, median survival time (MST) was not different in surgery alone (n = 52) and adjuvant S-1 (n = 100) (31.3 vs 35.8 months, p = 0.41). Propensity score matching yielded 48 patients (24 patients in each group). After matching, baseline characteristics were well balanced between the two groups. Survival in patients with adjuvant S-1 was significantly better than in surgery alone patients (MST: 50.3 vs 15.4 months, p = 0.002). Cox proportional hazard analysis revealed adjuvant S-1 treatment was selected as independent prognostic factor (HR: 0.38, 95%CI: 0.18-0.76, p = 0.006), as well as lavage cytology (HR: 3.9, 95%CI: 1.8-8.9, p < 0.001). Conclusions: Adjuvant chemotherapy with S-1 may have a strong impact on survival in type 4 and large type 3 gastric cancer patients. The efficacy of this treatment will be further demonstrated in the future clinical trials.