Antitumor activity (increase in lifespan and cure) was greater for ifosfamide (IFO) in several experimental tumors, some of which were primarily resistant to cyclophosphamide (CYC). IFO has been shown to be active in anthracycline-resistant and in adriamycin/cisplatin-resistant sublines of an Ehrlich ascites tumor, as well as in tumor cells primarily resistant to CYC. The few comparative controlled clinical trials available suggest superior single-agent activity of IFO compared with CYC in soft tissue sarcoma and ovarian cancer. Combination chemotherapy with IFO has been effective in second-line treatment of sarcomas, malignant lymphomas, lung cancer, and testicular cancer, most of them pretreated with or refractory to CYC. Although it is difficult to obtain clinical proof that there is no cross-resistance between IFO and CYC, IFO has been shown to be active in multirefractory malignant lymphomas, in small cell lung cancer not responding to adriamycin, vincristine, and etoposide, and in soft tissue and bone sarcomas. Testicular cancer and pancreatic cancer are some of the tumors in which IFO activity is currently being evaluated and in which CYC has so far failed to show sufficient clinical activity. More comparative controlled clinical trials are needed in ovarian cancer, breast cancer, malignant lymphomas, sarcomas and cervical cancer, in which IFO has already shown sufficient single-agent activity. Due to its lower level of cross-resistance with a variety of heterocyclic products, but also with other alkylating agents, in addition to its use in induction chemotherapy, IFO is an important second-line agent in many clinical situations.