Background: Mutations of the KRAS gene were identified as a predictive marker in mCRC for anti-EGFR antibody. Previously reported data suggest that the longer overall survival (OS) observed with bevacizumab (BV) treatment in mCRC is independent of alterations in the KRAS status. We analized efficacy of BV combined irinotecan and S-1 (IRIS/Bev) in mCRC relative to KRAS status. Methods: In the retrospective analysis (n=53) of patients who participated in the Phase II trial of IRIS/Bev, additional statistical analyses were done with data from KRAS mutational analyses. In this trial, eligible patients had to have mCRC with a confirmed diagnosis of adenocarcinoma, an age of >20 years, and no history of prior chemotherapy. S-1 40-60 mg twice daily p.o. was given on days 1-14 and irinotecan 100 mg/m2 and bevacizumab 5 mg/kg i.v. were given on days 1 and 15 of a 28-day cycle. The Response Evaluation in Solid Tumors (RECIST) criteria version 1.0 was used to assess tumor response. The Kaplan–Meier method was used to determine Progression-free survival (PFS) and OS. Log-rank test was used to compare with mutant or wild-type KRASin terms of PFS and OS. All statistical tests were performed using SPSS. Results: The target number of 53 patients was enrolled as of March 2009. KRAS status was assessed in 43 patients (wild = 27, mutant = 16). Response rate was 63.0% with wild-type and 68.8% with mutant-type KRAS, that was not significant (p=0.752). The median Progression-free survival was 17.1 months with wild-type and 22.7 months with mutant-type KRAS, that was not significant (p=0.531). And median OS was 49.0 months with wild-type and 38.0 months with mutant-type KRAS, that was not significant(p=0.906) as well. Conclusions: IRIS/Bev provides clinical benefit in patients with mCRC expressing either mutant or wild-type KRAS. Randomized control trial comparing this regimen with oxaliplatin containing regimen(XELOX or mFOLFOX6 plus bevacizumab: TRICOLORE study) is already started. Comparison of the efficacy of KRAS status is also planned in this study. Clinical trial information: NCT00569790.