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Background:During orthodontic tooth movement, the early response of periodontal tissues to mechanical stress is an acute inflammatory one. This study uses a longitudinal design to examine lactate dehydrogenase (LDH) activity in gingival crevicular fluid (GCF) to determine if GCF LDH can be used as a diagnostic aid in monitoring tooth movement and tissue response during orthodontic treatment. Methods:Seventeen patients (mean age: 16.1 years) participated in the study. Each patient was undergoing treatment for distal movement, and an upper first molar served as the test tooth (TT), while the contralateral (CT) and antagonist (AT) teeth were used as controls. The CT was included in the orthodontic appliance, but was not subjected to the distal movement; the AT was free from any orthodontic appliance. The GCF around the experimental teeth was harvested from both mesial and distal tooth sites immediately before appliance activation, and on days 7, 14, and 21. Clinical gingival conditions were also recorded. Results:Gingival crevicular fluid LDH activity was significantly elevated in all sites of the TT and CT, as compared to the AT, where LDH activity remained at the baseline level throughout the study. Enzyme activity levels were also greater in the TT than in the CT, and in the compression sites. Conclusions:Our results suggest that GCF LDH levels reflect the biological activity that takes place in the periodontium during orthodontic movement, and therefore they can be used as a diagnostic tool for monitoring for correct orthodontic tooth movement in clinical practice. J Periodontol 2005;76:411‐417.