We studied two children with recurrent cancer, who during treatment with ifosfamide developed severe encephalopathy characterized by coma of several days duration. Ifosfamide was used as a single drug without mesna as an uroprotector. A 1-hour infusion of ifosfamide (1.8 g/m2) was given on days one and two of a planned 5-day course. Encephalopathy was associated with severe electroencephalographic abnormalities, e.g. slowing in delta range in one patient and electrographic seizures in another. Both, clinical and electroencephalographic features of encephalopathy were reversible.