In 409 sufferers from various malignant tumours, we used the cytostatic Ifosfamide (ASTAZ4942) in fractionated doses. The total i.v. dose averaging 300 mg/kg bodyweight, was either spread over 5 consecutive days (5 X 60 mg/kg i.v.) or over 10 consecutive days (10 X 30 mg/kg). At the same time, most patients were irradiated, the radiation dose usually being only one tenth the antitumour dose. Infections and electrolyte imbalance were first treated before Ifosfamide therapy was instituted. Cases of advanced cerebral sclerosis, thrombopenia below 75,000/cmm, cerebral metastases, impaired renal function and inadequate cooperation of the patient were excluded from the studies. To prevent and control side effects, various premedications and adjuvants are required: Antiemetics, prevention of cystitis and infections, cardiovascular agents etc. Corticosteroids are contraindicated. Out of 360 assessable patients 101 had a full remission, 150 a partial remission, 79 were failures; 30 cases were not evaluated. Good results were seen especially in ovarian carcinoma, mammary carcinoma and microcellular bronchial carcinoma. Particularly striking is the drug's effectiveness in testicular tumours including teratomas, osteosarcomas, chondrosarcomas and myosarcomas as well as in some adenocarcinomas of the gastro-intestinal tract, particularly pancreatic carcinoma. In lymphoreticular tumours and haemoblastoses, its potency is less pronounced. The side effects of Ifosfamide are the same as those of other alkylating agents. They are reversible and can usually be controlled or even avoided by adequate preventive measures. In the order of incidence we observed: Alopecia, leukopenia, fall in haemoglobin, cystitis, intercurrent infections, nausea and vomiting as well as cerebral disorders. Since haemorrhagic cystitis considerably interferes with Ifosfamide treatment, its prevention is of essential importance. Because of possible complications and specific premedication and adjuvant measures for their control, this type of treatment should for the present only be carried out by oncologists or special cancer centres.