We describe herein a case of sporadic retroperitoneal aggressive fibromatosis. A 54-year-old man with a palpable abdominal mass was referred to our hospital for investigation and treatment. Abdominal ultrasonography and computed tomography revealed a solid mass with relatively well-defined borders in the left abdominal retroperitoneum. At surgery, a large tumor (14 x 13 x 11 cm) was found, arising from the retroperitoneal space and involving the wall of jejunum. Complete removal was achieved. Histological examination of the resected specimen revealed the presence of changes consistent with aggressive fibromatosis. The patient was not scheduled for any further treatment. Two years after surgery, the patient is without any signs of recurrent disease. Although extremely rare, aggressive fibromatosis should be considered in the differential diagnosis of retroperitoneal masses. Complete surgical resection with negative pathological margins remains the first line management of these neoplasms. Careful follow-up is indicated because recurrence may occur every time after surgery.