Objectives: to determine the diagnostic validity of the mandibular cortical index (MCI) for the diagnosis of osteoporosis in peri- and post-menopausal women. Methods: Volunteer female subjects in the 45 to 70 year age band, recruited from four European centres, underwent dual x-ray energy absorptiometry of the hip and lumbar spine, to provide a gold standard diagnosis of osteoporosis, and a DPR examination. Five observers, all oral radiologists but of different experience, made an assessment of porosity of the cortex of the lower border of the mandible using MCI. Results: Data of 661 subjects (mean age 54.8y; sd = 6.19y) were available for analysis, with 140 (21.2%) being classified as having osteoporosis. MCI data for each observer were dichotomosed so that MCI grade 3 indicated a positive test result and grades 1 and 2 a negative test result. The sensitivities and specificties of MCI for osteoporosis diagnosis were determined: Osteoporosis at any site Osteoporosis at femoral neck Observer Sensitivity (%) Specificity (%) Sensitivity (%) Specificity (%) 1 24.8 93.3 27.3 91.3 2 23.4 93.2 24.2 91.3 3 23.4 91.5 24.2 89.8 4 20.6 99.8 22.7 91.8 5 19.1 91.1 22.7 90.3 Interobserver repeatability (using weighted Kappa) showed a range of 0.183 to 0.780, with a median value of 0.467. This median indicated moderate agreement. Conclusions: MCI had low sensitivity but high specificity for diagnosis of osteoporosis. This high specificity might prove to be more appropriate for use in primary dental care than using a different diagnostic threshold. This work was supported by a research and technological development project grant from the European Commission FP5 'Quality of Life and Management of Living Resources' (QLK6-2002-02243).