A report is presented here of the total extirpation of a teratoma located in the third ventricle of a boy of eight. Since the tumour was made up of mature tissues (adult teratoma), its extirpation may be regarded as a cure. Although there are a few exceptions to the rule, other tumours located in the third ventricle of brain are mostly malignant, and they penetrate the tissues which constitute the wall of the third ventricle. For that reason the effects of a radical operation are doubtful. The adult teratoma is an exception. A tumour of this kind should be extirpated as a whole and removed from the third ventricle of brain.