BACKGROUND Gas in the portal venous system, or hepatic portal venous gas, is a rare occurrence associated with ischemic colitis, inflammatory bowel disease, or any cause of bowel perforation, including from a necrotic tumor. This report presents the case of a 72-year-old man with diabetes who had carcinoma of the ileocecal region, sepsis due to Klebsiella pneumoniae, and hepatic portal venous gas. CASE REPORT A 72-year-old man with ileocecal cancer was admitted to our hospital for preoperative diabetes control. He developed a fever and septic shock, without abdominal symptoms or signs of peritoneal irritation. Klebsiella pneumoniae was detected in blood cultures. Abdominal ultrasonography showed hepatic portal venous gas, and a simple computed tomography scan revealed gas in the vasculature and hepatic portal vein in the lateral segment, which led us to believe that the ileocecal mass was the source of infection, and emergency surgery was performed. The patient was discharged from the hospital on postoperative day 34 with good progress despite dehydration due to high-output syndrome. CONCLUSIONS Sepsis due to necrosis of ileocecal cancer is often difficult to diagnose because it is not accompanied by abdominal symptoms, as in our case. However, abdominal ultrasound is useful because it allows for a broad evaluation. This report has demonstrated and highlighted that the findings of hepatic portal venous gas on imaging should be regarded seriously, requiring urgent investigation to identify the cause and commence treatment in cases of infection or sepsis.