Young, systemically healthy subjects may suffer from early-onset forms of periodontitis characterized by the presence of localized deep vertical bony defects. The aim of this study was to compare the healing response after guided tissue regeneration (GTR) treatment of similar intrabony defects in patients affected by early-onset and chronic adult periodontitis. Twenty systemically healthy, nonsmoking subjects were enrolled in the study; 10 were affected by early-onset periodontitis (EOP) and 10 by chronic adult periodontitis (CAP). In each subject, only one deep vertical bony defect (intrabony component > 4 mm, probing attachment level ¡Ý 8 mm) was treated according to the principles of GTR therapy with titanium-reinforced e-PTFE membranes. At the time of the surgery and at the 1-year follow-up, a microbiologic test for the identification of the main periodontopathogens was performed in each of the treated sites. There was no statistically significant difference at 1 year in the amount of clinical attachment gain (P = .4), reduction of probing pocket depth (P = .3), or increase in gingival recession (P = 1.0) between EOP and CAP patients. The 1-year microbiologic results demonstrated the complete disappearance of the putative periodontopathogens fromYoung, systemically healthy subjects may suffer from early-onset forms of periodontitis characterized by the presence of localized deep vertical bony defects. The aim of this study was to compare the healing response after guided tissue regeneration (GTR) treatment of similar intrabony defects in patients affected by early-onset and chronic adult periodontitis. Twenty systemically healthy, nonsmoking subjects were enrolled in the study; 10 were affected by early-onset periodontitis (EOP) and 10 by chronic adult periodontitis (CAP). In each subject, only one deep vertical bony defect (intrabony component > 4 mm, probing attachment level ¡Ý 8 mm) was treated according to the principles of GTR therapy with titanium-reinforced e-PTFE membranes. At the time of the surgery and at the 1-year follow-up, a microbiologic test for the identification of the main periodontopathogens was performed in each of the treated sites. There was no statistically significant difference at 1 year in the amount of clinical attachment gain (P = .4), reduction of probing pocket depth (P = .3), or increase in gingival recession (P = 1.0) between EOP and CAP patients. The 1-year microbiologic results demonstrated the complete disappearance of the putative periodontopathogens from all surgically treated sites in both patient groups. The results of the study demonstrated that deep intrabony defects in patients with EOP can be successfully treated by means of GTR procedures and that the suppression of periodontopathogens under threshold values can be maintained for at least 1 year, provided that the patient is enrolled in a maintenance program consisting of recalls for professional tooth cleaning and reinforcement of self-performed oral hygiene measures at 1-month intervals. (Int J Periodontics Restorative Dent all surgically treated sites in both patient groups. The results of the study demonstrated that deep intrabony defects in patients with EOP can be successfully treated by means of GTR procedures and that the suppression of periodontopathogens under threshold values can be maintained for at least 1 year, provided that the patient is enrolled in a maintenance program consisting of recalls for professional tooth cleaning and reinforcement of self-performed oral hygiene measures at 1-month intervals.