INTRODUCTION: Mesothelioma is an uncommon, but feared malignancy. Its peritoneal variant is an even rarer entity and is classified based on 3 histologic subtypes: sarcomatoid, epithelioid, and biphasic. The sarcomatoid variant carries the worst prognosis. CASE DESCRIPTION/METHODS: A 26-year old female presented with a 6-month history of recurrent abdominal pain, nausea, loose stools, and unintended weight loss. Her past medical history was significant for squamous cell carcinoma (SCC) of the tongue which was treated with surgical resection including lymph node dissection with plans for adjuvant chemotherapy/radiation after evaluation of her gastrointestinal symptoms. She underwent extensive workup that included labs, stool studies, EGD, colonoscopy, and MR enterography; resulting in a presumed diagnosis of Crohn's disease based on endoscopic, but not histologic, evidence of ileitis. She was referred to our inflammatory bowel disease center, and a plan to repeat colonoscopy and ileoscopy with biopsies was made after seeing our GI and surgical subspecialists. Before outpatient workup could be completed, the patient presented emergently with nausea, reduced flatus, and reduced stooling. CT showed distal small bowel obstruction due to abnormal thickening of 5–10 cm of the terminal ileum with mucosal enhancement and extensive wall edema and inflammation of the adjacent mesenteric fat. Inpatient colonoscopy with ileoscopy revealed mild erythema and edema of the terminal ileum and sigmoid colon. A non-traversable, smooth appearing stricture was encountered 5 cm proximal to the ileocecal valve. The endoscopic appearance was not consistent with Crohn's disease. Ileal biopsies revealed prominent lymphangiectasia and associated erosion of overlying mucosa, and sigmoid colon biopsies were normal. Due to worsening symptoms, a diagnostic laparoscopy was performed, revealing an extraluminal, 6–7 cm that compressed the terminal ileum and was adherent to the sigmoid colon. She underwent an ileocecal resection with primary anastomosis. Final pathology revealed metastatic malignant mesothelioma of the sarcomatoid variant. She recovered well from surgery and is undergoing oncologic care. DISCUSSION: Our case demonstrates a unique presentation of a rare, clinically challenging entity that mimicked a typical presentation of Crohn’s disease.Figure 1.: CT Abdomen/Pelvis.Figure 2.: Terminal Ileal Biopsy with dilated lymphatic spaces and overlying eroded mucosa- 4x.Figure 3.: Terminal Ileal Biopsy with dilated lymphatic spaces and overlying eroded mucosa- 10x.