e16038 Background: Chemotherapy (CTX) is the standard of care for incurable advanced gastric cancer (AGC). REGATTA trial, which investigated survival benefit of reduction gastrectomy (Gx) in patients (pts) with AGC with a single non-curable factor, failed to demonstrate the superiority of Gx followed by CTX over CTX alone. However, there was a significant interaction between the treatment effect and tumor location in the subset analysis. In addition, treatment effect appeared to be different between Japan and Korea. So, the aim of this supplementary analysis was to explore the subgroups for which volume reduction Gx was beneficial with special reference to the tumor location and country in REGATTA trial. Methods: REGATTA trial was a randomized phase III trial at 44 institutions in Japan, Korea, and Singapore. Patients with AGC with a single non-curable factor were randomly assigned in each country to CTX alone or Gx plus CTX. The primary endpoint was overall survival (OS). In this analysis, OS was compared according to tumor location, country, and operative procedure. Results: A total of 175 pts (95 in Japan, 80 in Korea) was randomized to CTX alone (86 pts) or Gx followed by CTX (89 pts). MST was 16.6 months in CTX alone and 14.3 months in GX plus CTX (HR 1.09, 95% CI: 0.78–1.52, p = 0.70) (primary analysis). In subgroup analysis according to the country, OS curves were separated between the arms, worse in Gx plus CTX arm in Japan (HR 1.32, 95% CI: 0.85-2.05), whereas, that was not, almost similar between the arms in Korea (HR 0.85. 95% CI: 0.52-1.40). HRs according to tumor location were 2.23 in upper third, 0.95 in middle third, and 0.63 in lower third, respectively. OS tended to be better in pts who received distal GX compared with CTX alone (HR = 0.69), and worse in pts who received total GX compared with CTX alone (HR = 1.34). This trend was more remarkable in Korea than in Japan. Conclusions: Primary CTX is a standard of care for AGC; however, it would be still an open question whether volume reduction Gx have a survival benefit when the tumors located at the distal stomach for which the pts receive distal Gx especially in Korean population. Clinical trial information: UMIN000001012.