Objectives: To determine the diagnostic validity of the width of the inferior mandibular cortex on dental panoramic radiographs (DPRs), as measured directly by observers, 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 manual measurements of width of the mandibular lower border cortex below the mental foramina bilaterally. 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. Az values are given below: Osteoporosis at any site Osteoporosis at femoral neck Observer Az (se) 95% CI Az (se) 95% CI 1 0.782 0.748-0.813 0.804 0.771-0.833 2 0.766 0.731-0.799 0.757 0.722-0.791 3 0.756 0.721-0.788 0.790 0.757-0.821 4 0.746 0.711-0.779 0.762 0.727-0.794 5 0.710 0.673-0.744 0.752 0.718-0.785 A diagnostic threshold of 3mm resulted in a sensitivity of 50.7% and a specificity of 80.4% (data for median observer, 2). Mean within-subject variance for the five observers was 0.126mm (sd = 0.355mm). Repeatability is the difference between two measurements made by any pair of observers for the same subject and was expected to be less than 0.983 mm for 95% of pairs of observations. Conclusions: Direct measurement of mandibular cortical width was diagnostically effective in diagnosis of osteoporosis. However, repeatability may be a problem in clinical use. 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).