PURPOSE: About 80% of the intra-abdominal abscesses usually occur as a consequence of a visceral organ‟s infection. Primary omental disease especially infection or abscess rarely occurs. We emphasize the rarity of this case in association with other similar cases in literature as primary omental torsion, paracolic pseudotumor omentitis, actinomycosis, foreign body reaction etc. We also documented the difficult preoperative diagnosis and the presence of high concentration of the tumor marker CEA in the abscess fluid, which, to our knowledge, represents a unique report. METHODS: A 59-year-old white male presented to the emergency Department with symptoms and clinical signs of peritonitis. The preoperative imaging revealed a large cystic formation 5x4x6.5cm, with thickened wall inside the peritoneal cavity, but it was unable to estimate the diagnosis. RESULTS: During exploratory laparotomy a sizable abscess with pseudocapsule was found inside the greater omentum, with no other concomitant surgical finding as foreign body or surgical material associated with antecedent surgery. Omentectomy with culture of the abscess fluid and pathology identification of this intraabdominal inflammatory mass was the ultimate surgical intervention. CONCLUSĠON: The differential diagnosis of inflammatory masses in the sense of acute abdomen should take into account the presence of primary omental abscess. Serum concentrations of tumor markers (for example, CEA, CA 19-9, and CA 125) were elevated in several patients with abdominal cysts and inflammatory fluids, especially in bronchial secretions. [ABSTRACT FROM AUTHOR]