Background Orthodontic therapy has been suggested to lead to an improved periodontal status through mechanisms such as increased ease of plaque removal and reduced occlusal trauma. The objective of the authors' systematic review was to compare contemporary orthodontic treatment with no intervention, by means of evaluating periodontal outcomes measured after end of treatment. Methods. The authors completed electronic searches in eight databases (1980-2006) and hand searches in six dental journals (1980-2006). They extracted data using standardized forms and calculated weighted mean differences. Results. Weak evidence from one randomized study and 11 nonrandomized studies suggested that orthodontic therapy was associated with 0.03 millimeters of gingival recession (95 percent confidence interval [CI], 0.01-0.04), 0.13 mm of alveolar bone loss (95 percent CI, 0.07-0.20) and 0.23 mm of increased pocket depth (95 percent CI, 0.15-0.30) when compared with no treatment. The effects of orthodontic therapy on gingivitis and attachment loss were inconsistent across studies. Conclusions. This systematic review identified an absence of reliable evidence describing positive effects of orthodontic treatment on periodontal health. The existing evidence suggests that orthodontic therapy results in small detrimental effects to the periodontium.