During orthodontic treatment, the early response of periodontal tissues to mechanical stress involves several metabolic changes that allow tooth movement. Many studies have evaluated such modifications by analysis of various host metabolites released into the gingival crevicular fluid (GCF). This study used a cross-sectional design to examine the lactate dehydrogenase (LDH) activity in GCF to assess whether GCF LDH can be proposed as a sensitive marker for periodontal tissue modifications during orthodontic tooth movement. Thirty-seven subjects, 16 males and 21 females (mean age 18.7 years, range 14.0 to 26.7 years), participated in this study. Each subject underwent a session of professional oral hygiene and received oral hygiene instructions; 2 weeks later, a fixed orthodontic appliance was placed on the maxillary arch. A randomly selected maxillary canine was considered as the test tooth, and its antagonist, which had no appliance, was used as the control tooth. From 2 to 12 weeks after orthodontic appliance placement, GCF was harvested from both experimental teeth at the mesiobuccal angle, for GCF volume and LDH activity determinations. Clinical monitoring consisted of recording supragingival plaque presence, bleeding on probing, and probing depth at the same collection sites. The results showed that no differences in clinical conditions and GCF volume occurred between the experimental teeth. On the contrary, GCF LDH activity in the test teeth was significantly greater than that of the control teeth (P <.01). Moreover, no differences were found in the enzymatic activity between the sexes by experimental tooth, and no significant correlation was present between GCF LDH activity and patients' ages within experimental teeth. Our enzymatic results initially indicated a possible role of GCF LDH during the early phases of orthodontic treatment and therefore warrant further study as a possible diagnostic tool for tissue response during orthodontic treatment.