In a retrospective study, the authors evaluated the effectiveness of periodontal treatment in preventing tooth loss in patients with moderate to advanced periodontitis. Patients had received maintenance therapy for an average of 12.5 years. Of the 2,899 teeth present after active treatment, 152 were lost to periodontal disease and 68 were lost to other causes during maintenance therapy. The authors evaluated causes and patterns of tooth loss after surgical vs. nonsurgical treatment. Surgery did not significantly improve tooth retention in the high-risk patients. This study and other retrospective studies suggest that tooth loss may be related more to the type of periodontal disease present than to the treatment rendered.