A retrospective analysis of all MMRD mCRC patients that were treated with either Pembrolizumab, or Ipilimumab and Nivolumab....Fifteen patients had peritoneal metastasis, of them 13 (87%) responded to treatment (HR = 0.24; 95% CI = 0.05-1.19; p = 0.081). Median CEA at treatment initiation was 2 (IQR 1-4) in responders compared to 32 (IQR 1-70) for non-responders (HR = 5.27; 95% CI = 1.31-21.2; p = 0.019)....Liver metastasis predict poor response to immunotherapy in MMRD mCRC patients while peritoneal metastasis and low CEA level predict response to immunotherapy.