The pin-bone interface is the weakest link in the mechanical stability of external skeletal fixation. In this investigation, a canine model was used to characterize the nature of cortical bone reactions at the pin-bone interface. Unilateral external fixators were applied to the tibiae of 61 dogs using six tapered cortical half-pins. The pins were inserted after predrilling both cortices, and pin insertion torque was measured. A transverse or oblique osteotomy was performed in each tibia and stabilized under different gap conditions. Unrestricted weight bearing was allowed immediately after surgery. Sequential functional evaluations of the animals and biweekly roentgenograms of the pin tracks were done. Pin removal torques were recorded at the time the animals were killed at different time periods. Pin tracks were analyzed using quantitative tetracycline histomorphometry and microradiography. The results showed that cortical bone undergoes extensive creeping substitution around external fixation half-pins. New bone accounted for approximately 43% of the intracortical space along the pin track, and cortical bone porosity showed a fourfold increase compared with intact bone value. This cortical bone remodeling resulted in a timerelated decrease of pin removal torque (p< 0.001). In inherently unstable oblique osteotomies, and less in stable rigidly fixed transverse osteotomies, immediate postoperative weight bearing caused bone thread resorption and adverse cortical bone remodeling at the entry cortex of external fixation half-pins. The unicortical loosening of half-pins that became evident during the first month of fixation obviously represents a consequence of micromotion and local bone yielding failure caused by high dynamic stresses of the pin-bone interface. Effective precautions should be taken to reduce such stresses.