The authors report a rare case of a 10-year-old girl with craniopharyngioma which showed malignant change after the first operation and irradiation. In June 1981, the patient complained of headache, nausea and vomiting. CT revealed obstructive hydrocephalus due to the calcified mass lesion which extended to the third ventricle. In order to alleviate the high intracranial pressure, the right ventriculo-peritoneal shunt was first settled and after that, partial removal of the tumor was performed. The pathological diagnosis of the specimen was typical adamantinomatous type of craniopharyngioma without any findings of malignancy. After this operation irradiation was performed. The tumor almost disappeared and the patient was discharged from the hospital and went to school, showing some signs of panhypopituitarism. In May 1984, she complained of decreased left visual acuity, right temporal anopsia, headache, nausea and vomiting. CT revealed recurrence of the tumor which obstract the foramen of Monro bilaterally. As an emergency measure, the left ventriculo-peritoneal shunt was added and the state of the patient became recovered. In order to improve decreased visual acuity, the tumor located around the optic nerves and over the frontal base was removed in June 1984, resulting in partial improvement of visual acuity bilaterally. The pathological examination of the second specimen showed, in addition to the part of adamantinomatous type of craniopharyngioma which was the same as before, the existence of thick layer of stratified large atypical cells which partially covered the cyst wall and partially invaded into the surrounding tissues. The pathological diagnosis was poorly differentiated squamous cell carcinoma with craniopharyngioma of 'adamantinoma' type.(ABSTRACT TRUNCATED AT 250 WORDS)