The purpose of this in vivo study was to verify the accuracy of cone beam computed tomography (CBCT) in the measurement of horizontal periodontal bone defects. Six patients with periodontitis were selected, and eight maxillary molars were assessed. A total of 72 measurements were performed. The images were obtained using CBCT, and measurements were performed using the appropriate software. Periodontal bone defects were measured during the surgical intervention using the cementoenamel junction as the reference point. There were no statistically significant differences between clinical and CBCT measurements (P > .05). CBCT accurately reproduced the clinical measurement of horizontal periodontal bone defects.