The toxicity profile of the alkylating anti-cancer drug ifosfamide is quite well known. However, up to now only a few cases of hypersensitivity as an attributed adverse effect of ifosfamide have been described in literature. This article describes the history of a woman suffering from a primary peripheral neuroectoder mal tumor (PNET) who was admitted to our hos pital for chemotherapy. Treatment was given ac cording to the EICESS-protocol (European Intergroup Cooperative Ewing's Sarcoma Study) with the following anti-cancer drugs: etoposide, doxorubicin, vincristine, and ifosfamide/mesna. Af ter ifosfamide administration an acute hypersensi tivity reaction was seen consisting of redness in the face and on the left arm, back and chest, and bronchoconstriction. Subsequent intravenous ad ministration of the anti-histaminic clemastine (2 mg) and dexamethasone (5 mg) before ifosfamide administration on a second occasion did not pre vent these reactions. After substituting cyclophos phamide for ifosfamide in consecutive courses no untoward reactions occurred.