Objective: This study evaluated the predictability of periodontal treatment as measured by tooth loss in 114 patients. Method and materials: The patients were selected for the study if they had been treated for moderate-to-advanced periodontitis and had received continuous periodontal maintenance every 3 to 6 months for 5 years or longer. The patients averaged 53 years of age, were equally divided by gender, and were maintained an average of 12.5 years. Patients were retrospectively assigned, according to the number of teeth lost following active periodontal treatment, to the well-maintained group (96 patients, 0 to 3 teeth lost), downhill group (15 patients, 4 to 9 teeth lost), or extreme downhill (three patients, 10 to 23 teeth lost) group. Teeth were retrospectively assigned a questionable or hopeless prognosis. Results: Of the 2,899 teeth present during maintenance care, 378 had a questionable prognosis because of furcation involvement, and 63 (16.7%) of these teeth were lost. Of the 529 nonfurcated teeth that had a questionable prognosis, 68 (12.9%) were lost during the maintenance phase of treatment. Ninety-dix teeth were given a hopeless prognosis, and 32 (33.3%) of these teeth were lost. Surgical treatment of teeth with questionable or hopeless prognosis did not improve the retention of these teeth in the downhill and extreme downhill groups. Conclusion: The prognosis of a periodontally compromised tooth in individual patients is difficult to predict accurately. A hopeless prognosis is more accurate than a questionable prognosis in predicting tooth loss in these patients.