Objective To evaluate the accuracy and reliability of cone-beam computed tomography (CBCT) in the diagnosis of periodontal bone defects in the elderly.Methods The 38 teeth of 27 cases were collected and scanned both with CBCT technology and periapical radiography (PA).Periodontal bony defects were identified and linear measurements were made for all defects.All cases were treated with periodontal flap surgery with the bony defects and their linear measurements detected during operation as the gold standard.Results All periodontal bony defects were identified with CBCT with its accuracy rate of 100.0％ (72/72),but the buccal and lingual defects could not be measured by PA with its accuracy rate of 62.5％ (45/72).Linear measurements for all defects revealed no statistical differences between CBCT and direct measurement [(5.516 ± 0.393) mm vs.(5.385± 0.415) mm,t=1.948,P＞0.05],but there was a significant difference in linear measurement between PA and direct measurement [(6.311±0.439) mm vs.(5.411±0.418) mm,t=9.956,P＜0.05].Conclusions CBCT can offer the diagnostic information for periodontal bone defect from three dimensional direction in the elderly,provide objective and accurate assessment of alveolar bone resorption,and has higher accuracy and reliability.