The repair process of external root resorption (ERR) and the role of retention mechanics in enhancing ERR repair were studied on eight Macaca fascicularis monkeys that were divided equally into short- and long-term groups. Six monkeys received palatal expansion appliances, and two received sham appliances. The short-term group received active treatment. The long-term group received additional retention (4 months) and relapse (2 months) treatment periods with biweekly injections of individual vital dye per phase, i.e., procion red H-8B and violet H-3R (80 mg/kg B.W.), respectively. Histomorphologic examinations included scanning electron microscopy, light, and ultraviolet microscopy. The short-term group demonstrated penetrated resorption with pulp exposure at sites with initial deficiency of the protecting odontoblastic layer (apical zone, nutrition canal). The long-term group showed two forms of ERR repair: (1) Nonfunctional retarded repair cementum, comprised of overlapped incremental lines and deprived of extrinsic fibers, was delineated in severe pulp exposure. The pulp/dentin complex showed intense incorporation of procion dye in the dentinal tubuli, conceivably related to a defense response in the form of sclerotic dentin. (2) Functional rapid repair cementum, comprised of discriminated incremental lines mainly of mixed cellular cementum, with a consistent pattern of five sequential phases: the lag phase (14 to 28 days), the incipient phase (14 days), the peak phase (14 to 28 days), the steady phase (42 to 56 days) and the retreating phase (70 days). Sharpey's fibers at functional ERR sites were scarce, never emerging from the dentinocemental junction, and not developing into principal fibers. The pulp/dentin complex showed an increase in pulp stones but no formation of tertiary dentin. The apical area responded by hypercementosis in the form of apical occlusion and a displaced pulp canal. The application of a fixed retention device is recommended in light of accelerated apposition of repair cementum during the retention period. However, increased formation of Sharpey's fibers during the relapse period might suggest a restricted duration in splinting therapy.