Clostridium septicum aortitis is a rare infection that has a strong association with occult colonic malignancy. To our knowledge, we report the 25th and 26th cases of C septicum aortitis in the English literature and make recommendations for its management. The first patient was a 75-year-old man who presented with abdominal pain. Computed tomography showed the presence of periaortic gas. He underwent aortic débridement and extra-anatomic bypass after blood cultures revealed C septicum. Four months after the initial presentation, he was readmitted with lethargy, found to have recurrent periaortic gas, and died. The second patient was a 76-year-old woman who presented with a 5-cm abdominal aortic aneurysm with surrounding retroperitoneal gas. She underwent emergency aortic ligation and retroperitoneal débridement. Her blood and intraoperative tissue cultures also grew C septicum. She had a prolonged postoperative course and ultimately died on hospital day 94. Both patients were found to have concurrent colon adenocarcinomas. C septicum aortitis is a lethal disease that necessitates prompt surgical intervention and appropriate antibiotic therapy. The strong association of C septicum with occult malignancy should prompt the astute clinician to undertake an exhaustive search for a neoplastic process.