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We report a previously undescribed fatal toxicity in a 54 years old female patient suffering from a locally advanced adenocarcinoma of the rectum. She received standard neoadjuvant chemoradiotherapy followed by an investigational preoperative chemoimmunotherapy regimen (mFOLFOX6 and nivolumab). On day 12 of the first cycle the patient became acutely ill with excruciating abdominal pain and a rapidly downhill course. At laparotomy severe retroperitoneal and mesenteric serositis was found with diffuse petechial lesions; no other gross pathology was evident; biopsies did not show vasculitis. The patient expired within 24 h from surgery. The possible interaction between radiotherapy to the pelvis and chemotherapy concomitantly with a checkpoint inhibitor resulting in unexpected toxicity is discussed. While serositis has been previously described as a possible side effect from checkpoint inhibitors, retroperitoneal and mesenteric serositis as an isolated finding has not been reported.