A 72-year-old female suffering from fever, chills, abdominal pain and tachypnea, was sent to our Emergency Room. Laboratory examination revealed a leukocytosis, primarily granulocytes, and respiratory alkalosis together with hypoxemia. The chest x-ray showed right parahilar haziness and the plain abdomen revealed an ileus. Localized LLQ ascites was detected by abdominal sonography. Under the impression of peritonitis with intraabdominal sepsis, an urgent laparotomy was undertaken. An unusual mesenteric abscess was found 30 cm below the Treitz ligment. The patient showed marked improvement after adequate surgical drainage and antibiotic treatment. This case emphasizes the need for a high degree of suspicion regarding the possibility of intraabdominal sepsis, especially in patients with unexplained septic manifestations.