The aim of this study was to measure the accuracy of porosity of the mandibular cortex on dental panoramic radiographs (DPRs) in diagnosis of osteoporosis, alone and in combination with a clinical risk index. Six hundred seventy-one women (45-70yr) were recruited in the study, and dual-energy X-ray absorptiometry of the hip and lumbar spine was performed. A clinical index of osteoporosis risk (OSIRIS) and a DPR were obtained for each subject. The cortical appearance on the DPR was classified using the mandibular cortical index (MCI) by 5 observers. receiver operating characteristic (ROC) curve analysis was performed with calculation of area under the ROC curve (AUC) and sensitivity and specificity at various thresholds. Complete data were available for 653 subjects, of whom 21.6% had osteoporosis. The AUC for OSIRIS was 0.838. When used alone as the diagnostic test, MCI AUC for the 5 observers ranged from 0.560 to 0.670, significantly less than OSIRIS. Intraobserver and interobserver repeatability of MCI assessment was inconsistent. We conclude that MCI has limited value for osteoporosis diagnosis, being most appropriate as a method of fortuitous case-finding.