The purpose of this histological study was to show the situation of poor resorption in the exposed root apex in the oral cavity in human deciduous incisors with severe apical periodontitis. The results showed abnormal resorption in the apical root dentin and partial repair by cementum deposition on the apical root dentin surface. It was suggested that resorption and repair of the infected root dentin took place in the early stage of the process on exposure of the root apex in oral cavity. Since the various reactions such as the defense mechanisms in vivo originated at the apical root and had resulted from excretion, it was thought that the realization of the exposed root apex needed considerable time and there were many environment factors which made up the situation of apical root, such as the level of microorganisms infection, the situation of apical periodontitis, the thickness of alveolar bone, the eruptive forces of permanent successors and so on.