Hemangiopericytoma is a rare and solitary solid tumor originating from pericytes, which are contractile pericapillary cells, first identified and described by Zimmermann in 1923. This neoplasm occurs most commonly in the lower extremities and retroperitoneum. The Authors describe the case of a patient affected with a giant retroperitoneal hemangiopericytoma. Echographic and computed tomographic scans have been fundamental in defining location and size of the neoplasm as well as its relations to the surrounding structures. However a definitive diagnosis was possible only by histologic examination, supported by the proper immunohistochemical stains. The Authors emphasize the difficulty to reliably predict the biological behaviour of these tumors and therefore the necessity of a follow-up of the patients for at least five years, even in the case of a histologically "benign" hemangiopericytoma.