Ewing sarcoma is the second most common bone tumor in childhood. Despite aggressive chemotherapy and radiotherapy strategies, the prognosis of patients with metastatic disease remains poor. We have recently reported that Ewing tumor cell proliferation was strongly inhibited by IFN-beta and to a lesser degree by IFN-alpha. Moreover, under IFN-beta treatment, some cell lines undergo apoptosis. Since the possibility of using IFNs for Ewing tumor treatments may be of interest, we have evaluated the efficacy of Hu-IFNs in a nude mice model of Ewing tumor xenografts. The results reported here show that human type I IFNs, Hu-IFN-alpha and Hu-IFN-beta impaired tumor xenograft take and displayed an anti-growth effect toward established xenografts. Furthermore, we have also shown that combined therapy with Hu-IFNs and ifosfamide (IFO), an alkylating agent widely used in high-dose chemotherapy of Ewing tumors, results in a strong antitumor effect. Pathological analysis showed that Hu-IFN-alpha/IFO and Hu-IFN-beta/IFO were characterized by a dramatic decrease in the mitotic index and marked necrosis, as well as extensive fibrosis associated with numerous calcifications. To our knowledge, this is the first demonstration of a potential antitumor effect of human type I IFNs and IFO on Ewing tumors, providing a rational foundation for a promising therapeutic approach to Ewing sarcoma.