Background: CO.26 was a phase 2 trial (2-sided α=0.1 and 80% power) that randomized 180 patients with refractory mCRC 2:1 to durvalumab + tremelimumab vs BSC with improved overall survival (OS) (HR 0.73, 90%CI 0.55-0.97, P=0.07). A Nanostring assay validated for use with FFPE was used to determine CMS for correlation with outcome. Methods: Archival FFPE from 163/180 (91%) of patients (pts) underwent RNA extraction and CMS subtyping. Cox proportional hazard models evaluated the prognostic and predictive impact of CMS on overall survival. Results: CMS distribution was skewed towards CMS4 (76%), with lower prevalence of CMS1 (2%), CMS2 (16%) and CMS3 (2%). There were 7/163 cases of indeterminate CMS (4%). Subgroup analysis was restricted to CMS2 and CMS4 based on sample size. With BSC alone, CMS2 showed trends to worse OS compared to all other patients pooled (HR 1.93, 90% CI 1.03-3.61, P=0.085), while CMS4 did not (HR 0.86, 90% CI 0.50-1.48, P=0.64). OS but not progression free survival (PFS) was improved with durvalumab + tremelimumab in the overall population. OS was improved with durvalumab + tremelimumab among patients with CMS2 tumors (HR 0.39, 90% CI 0.19-0.82, P=0.035) but not in patients with CMS4 tumors (HR 0.73, 90% CI 0.52-1.02, P=0.12) compared to BSC. Neither CMS2 (P-interaction=0.37) nor CMS4 (P-interaction=0.91) were predictive of OS benefit from durvalumab + tremelimumab compared to BSC. Disease control rate (DCR) trended to being better among CMS4 (24/85) than CMS2 cancers (1/15, OR 5.51, 90% CI 1.10-29.88, P=0.11) or CMS4 vs all non CMS4 cancers (1/21, OR 7.87, 90% CI 1.65-41.98, P=0.023) for patients on durvalumab + tremelimumab. PFS was not improved with durvalumab + tremelimumab in CMS2 (P=0.19) or CMS4 (P=0.29) cancers relative to BSC. Conclusions: In this trial of refractory colorectal cancer, we saw a shift in CMS subtype with more CMS4 than expected. Compared to CMS4, CMS2 showed stronger signals towards improved OS with durvalumab + tremelimumab but had a lower disease control rate. Differences in immune signaling by CMS may be important determinants of which component of immune regulation needs to be targeted in mCRC to improve outcomes. Clinical trial information: NCT02870920.