The aim of this study was to investigate the impact of midgut versus hindgut as the primary tumor site in patients with metastatic colorectal cancer (mCRC) receiving chemotherapy with FuFIRI or mIROX. We analyzed 423 patients from a phase III trial that randomized patients in a 1 : 1 fashion to either FuFIRI or mIROX. The cohort was grouped into midgut (n=82) and hindgut (n=341) primary tumors. The primary tumor site (midgut vs. hindgut) was correlated with parameters of treatment efficacy and survival. Our cohort comprised 82 patients presenting with primary midgut tumors and 341 with primary hindgut tumors. Tumors of midgut origin compared with hindgut origin were associated with inferior outcome. Objective response rate was 37 versus 43% (P=0.34), median progression-free survival was 6.0 versus 8.2 months (P=0.024, hazard ratio: 0.75), and median overall survival was 13.6 versus 21.8 months (P=0.001, hazard ratio: 0.65). Patients with midgut mCRC showed a clear trend toward inferior outcome in both study arms. However, the effect appeared less pronounced in the mIROX arm. Further datasets from large trials with various regimens are required as confirmation.