The research on the development and injury of the hard tissue of teeth was studied by the pedodontic departments of nine dental schools. We participated in this research and studied histopathologically hypoplasia of the enamel in particular. There were seven selected teeth which had obvious hypoplasia of the enamel. We conducted observations with the naked eye, replica, general microscopy, polarized light microscopy, fluorescence microscopy, and microradiography. Moreover we studied to classify these cases into three types, that is to say, the injury factors of whole body, locality, and unknown origin. The results obtained were as follows. 1) As for the injury factors for the whole body, the bottom of the enamel defects were located corresponding to the incremental line of Retzius in the case of enamel hypoplasia of the deciduous teeth. Also enamel hypoplasia was located near the dentino-enamel junction corresponding to the surface of hypoplasia. 2) As for the injury factors with respect to locality, the bottom of enamel defects were located corresponding to the incremental line of Retzius. The dentino-enamel junction was irregular and discontinuous, still more a part of that area, and the dentin protruded or was exposed. 3) As for the injury factors of unknown origin, hypoplasia was located from the surface of the enamel but it did not spread. Also the hypoplasia of the enamel did not correspond to the incremental line of Retzius. 4) Five cases had fluorescence lines in enamel or dentin.