Objectives: The first aim of this study was to evaluate intra-and interobserver agreement in evaluating the mandibular cortical index (MCI) in masked and unmasked panoramic radiographs. The second aim was to evaluate the effect of previous experience of MCI classification on the observers' performance. Methods: Three observers separately evaluated MCI, twice in masked and twice in unmasked panoramic radiographs, with 2 week intervals. The observations were repeated more than 9 months later. The second and third observers were inexperienced in evaluating MCI. Weighted kappa statistics were used to evaluate the inter- and intraobserver agreement. Results: In the first session of the study, the kappa values for intraobserver agreements in masked panoramic radiographs were between 0.565 and 0.789; in unmasked panoramic radiographs, they were between 0.420 and 0.732. The kappa values for interobserver agreement were between 0.131 and 0.392 in masked radiographs and between 0.043 and 0.225 in unmasked radiographs. In the second session of the study, the kappa values for intraobserver agreements in masked panoramic radiographs were between 0.586 and 0.786. The kappa values for intraobserver agreements in unmasked panoramic radiographs were between 0.558 and 0.781. The kappa values for interobserver agreement were between 0.146 and 0.510 in masked radiographs and between 0.174 and 0.568 in unmasked radiographs. Conclusions: Masking has no effect on the evaluation of panoramic radiographs for MCI. Previous experience causes higher intraobserver agreement. Intra- and interobserver agreement increases along with the increased experience in MCI evaluations. © 2009 The British Institute of Radiology.