Background: Primary mesenteric tumors are exceedingly rare and may thus pose a diagnostic challenge. They encompass both benign and malignant neoplasms as well as reactive and idiopathic tumefactive fibroinflammatory lesions. Method and results: A 70-year-old man who was diagnosed with sigmoid colon cancer was found to have a large non-homogeneous predominantly fatty retroperitoneal soft tissue mass on computerized tomography (CT) scan. The mass was attached to the aorta and have encased the inferior mesenteric artery and extended into the sigmoid mesocolon. Histological examination of the mass showed ill-defined lipoma-like mature fatty tissue traversed by paucicellular fibrous septa entrapping small nerves and containing remarkably increased venous blood vessels reminiscent of soft tissue angiomatosis without evidence of malignancy. The histological features were consistent with an angiolipomatous hamartomatous mesenchymal proliferation. Conclusions: Angiolipomatous hamartoma might be histogenetically related to soft tissue angiomatosis. To our knowledge, this case represents the first well documented lesion of this type at this location and must be distinguished from other fat-containing masses, particularly angiomyolipoma, sclerosing mesenteritis and mesenteric liposarcoma.