This study evaluated the use of guided tissue regeneration in preventing and treating bone loss after surgical removal of horizontally impacted mandibular third molars. A test group of 10 patients who were treated with expanded polytetrafluoroethylene membranes was compared with a control group of 10 patients who did not receive membranes. Patients were evaluated preoperatively and 3,6,9 and 12 months postoperatively for changes in gingival inflammation, clinical probing depth, probing attachment level, and recession. Within the limits of this study, the use of expanded polytetrafluoroethylene membranes greatly improved the clinical results.