Mafosfamide-cyclohexylamine is a new oxazaphosphorine derivative. It was chosen for phase-I clinical testing because of an expected higher therapeutic index and lack of complete cross resistance in animal tumors compared to cyclophosphamide. The schedule consisted of a single iv dose repeated every three weeks. The compound was found to cause as it's dose limiting toxicity severe pain along the injected vein and acute irritation of mucous membranes. The maximal tolerated dose was around 1000 mg/m2 given as a slow infusion over 2-3 hours. Hematological toxicity was mild. A limited phase-I study with the lysine salt of mafosfamide showed an identical type of toxicity. Mafosfamide given iv in a high-dose intermittent schedule is of little interest for further clinical trials. It is probable, that the severe venous pain and the mucosal irritation are caused by the high local concentration of 4-hydroxy-cyclophosphamide or by a metabolite. An oxazaphosphorine derivative undergoing slower hydrolysis therefore leading to lower active drug concentrations within the injected vein may be more promising.