The etiology of root resorption subsequent to replantation of incisors was examined in green Vervet monkeys. Cell damage to the root surface due to the extraction procedure was registered histologically 1 week after replantation. The topographical distribution of cell damage was then related to the development of root resorption in similar replanted teeth with longer observation periods. Histometric analysis showed that surface-, inflammatory-and replacement resorption was significantly related to certain topographical locations on the root surface. These surfaces represented the«corner»surfaces of the root, where the maximum damage presumably would occur during the extraction procedure. A positive and highly significant correlation between cell damage in the cementoblast layer and the presence of surface-, inflammatory-and replacement resorption was found: replacement resorption was associated with the greatest loss of vital cement-oblasts per unit root surface length. Furthermore, the distance from a potential resorption site to the nearest location on the root surface with a normal number of cementoblasts was found to be related to the type of root resorption. Thus, replacement resorption was found to be significantly related to the greatest distance from a site with normal numbers of cementoblasts compared to sites with no resorption or inflammatory resorption. Based on these findings, as well as previous experiments, a theory is presented for the etiology and pathogenesis of external root resorption after immediate replantation of mature teeth.