The use of at least one flavonoid of natural or synthetic origin in association with cyclophosphamide and/or methotrexate to increase the effectiveness of chemotherapeutic treatments used in human and veterinary medicine for the treatment of tumors is described, in particular in case of resistance to the chemotherapeutic agents currently in use. At least one flavonoid herein described is selected from the group comprising or, alternatively, consisting of rutin, oxerutin, diosmin, troxerutin and hesperidin.
