Background: In advanced gastric cancer (AGC), there are many reports about prognostic factors for overall survival (OS), and we have proposed a prognostic index using four prognostic factors (PS, number of metastatic sites, prior gastrectomy and ALP; Oncologist 2014) based on a phase III trial JCOG 9912 for the first-line treatment (Lancet Oncol. 2009). However, there is no report about prognostic factors for progression free survival (PFS). In this ancillary study, we explored whether prognostic factors are similar or not between OS and PFS. Methods: The subjects of this study were selected from the JCOG9912 which intended to confirm the superiority of irinotecan plus cisplatin (IP) and the non-inferiority of S-1 to5-FU for patients with AGC. Of all enrolled patients in JCOG9912, those who had target lesions and whose complete data were available were analyzed with multivariate analysis using Cox proportional hazard model. Results: 492 out of the 703 pts of JCOG9912 were analyzed, who received either 5-FU (n=163), IP (n=164) or S-1(n=165). The median PFS was 3.7 months for all the subjects, and 2.2 months for 5-FU, 4.9 months for IP and 3.8 months for S-1. Multivariate analysis in all 492 analyzed patients demonstrated seven independent prognostic factors for PFS (Table). Prognostic factors in each treatment were; sex (HR 1.66, 95%CI 1.11-2.49), PS (1.51, 1.04-2.18), Ca (0.39, 0.17-0.86), GPT (2.46, 1.30-4.66) and LDH (1.67, 1.16-2.48) in 5-FU, sex (1.77, 1.10-2.86) in IP, and Na (2.00, 1.01-3.99) and creatinine clearance (0.37, 0.15-0.93) in S-1. Conclusions: There were no common prognostic factors among the three treatment regimens. Prognostic factors for PFS may be different by treatment regimen, although further investigations with larger sample size are needed.