The primary tumor side is emerging as a prognostic factor for patients with liver metastatic colorectal cancer (mCRC). In a combined analysis of data from 2 randomized studies, the addition of selective internal radiation therapy to first-line chemotherapy in patients with mCRC was associated with statistically and clinically significant overall survival gains for patients with a right-sided primary tumor. Background: The primary tumor side is emerging as a major prognostic factor for patients with metastatic colorectal cancer (mCRC). We examined the survival data from 2 randomized studies to determine whether the outcomes differ between patients with mCRC with right-sided primary (RSP) tumors and those with left-sided primary (LSP) tumors after selective internal radiation therapy (SIRT) plus mFOLFOX6 (folinic acid [leucovorin], 5-fluorouracil, oxaliplatin) chemotherapy, versus chemotherapy alone. Patients and Methods: Separate and combined analyses were performed on the data from the SIRFLOX and FOXFIRE global trials, which compared chemotherapy plus SIRT with chemotherapy alone for patients with mCRC liver metastases. The primary tumor side data were prospectively collected. The principal outcome measure was overall survival (OS) stratified by treatment and primary tumor side. Results: In the combined analysis of all 739 patients enrolled, SIRT had no effect on OS (median OS, 24.3 vs. 24.6 months; hazard ratio [HR], 1.021; P =.810). For the 179 patients (24.2%) with a RSP tumor, OS was improved with the addition of SIRT (median, 22.0 vs. 17.1 months HR, 0.641; P =.008). The addition of SIRT was not associated with a significant difference in OS among the 540 patients with a LSP tumor (median, 24.6 vs. 26.6 months; HR, 1.120; P =.264). A test of treatment interaction by primary tumor side was statistically significant for RSP and SIRT (P =.002). Conclusion: The addition of SIRT for patients with RSP tumors, but not for those with LSP tumors, was associated with a statistically and clinically significant OS gain.