Background: Primary appendiceal diffuse large B-cell lymphoma (DLBCL) is exceptionally rare and most commonly presents as acute appendicitis with diagnosis typically established after appendicectomy. Case: A man in his 30s presented with right iliac fossa pain with several months of intermittent diarrhoea and unintentional weight loss. Inflammatory markers were normal. Contrast-enhanced CT demonstrated soft-tissue expansion of the distal appendix without typical periappendiceal inflammatory change and with contiguous ileocolic lymphadenopathy, including a necrotic node, raising suspicion for malignancy rather than acute appendicitis. CT chest was negative, tumour markers were normal, and colonoscopy was unremarkable. 18F-FDG PET demonstrated intense uptake in an ileocaecal mass and regional nodes, without distant metastases. As appendiceal adenocarcinoma was considered the leading preoperative diagnosis, the patient underwent laparoscopic radical right hemicolectomy with complete mesocolic excision, ileocolic anastomosis, and partial duodenectomy. Intraoperatively, there was a contained perforated tumour with a small abscess cavity and nodal disease adherent to the duodenum. Histopathology confirmed DLBCL (germinal-centre B-cell phenotype) with CD20, CD19, PAX5, CD10 and BCL6 positivity, Ki-67 ~90%, and no MYC rearrangement. He commenced R-CHOP nine weeks post-operatively; interim PET after cycle 4 demonstrated marked metabolic response (Deauville 3). Conclusion: Primary appendiceal DLBCL may present with atypical imaging features mimicking other appendiceal malignancies such as adenocarcinoma. Distinguishing these entities preoperatively remains challenging but is important, as management of DLBCL is primarily systemic and extensive surgical resection may not be required in selected cases. References: 1. Ayub A, Santana-Rodríguez N, Raad W, Bhora FY. Primary appendiceal lymphoma: clinical characteristics and outcomes of 116 patients. J Surg Res. 2017 Jan;207:174–80.