A multilocular-cystic and cavernous, retroperitoneal tumor was found in a 40-year-old man whose past medical history was unremarkable. On admission, he complained of a large and still growing intra-abdominal mass associated with dull pain and a low-grade fever. Laboratory findings revealed leukocytosis and C-reactive protein elevation, compatible with inflammation of the tumor. Percutaneous aspiration of the tumor was performed under transabdominal ultrasonographic guidance, and continuous drainage of fluid from within the tumor ameliorated his symptoms. From preoperative examinations, including radiological imaging, fluid aspiration, and endoscopy with biopsy, a diagnosis of retroperitoneal lymphangioma was made. Laparotomy revealed extensive adhesions between the tumor and both the duodenum and the pancreatic head. A pancreaticoduodenectomy was therefore performed. At 3-year follow-up, there was no sign of recurrence. Retroperitoneal lymphangioma is an uncommon disorder, and the cavernous type is extremely rare. The duodenal lesion was an important feature of the present case, and endoscopic biopsy of this lesion facilitated precise preoperative diagnosis of retroperitoneal lymphangioma.