Eroded or thin inferior cortex of the mandible detected on dental panoramic radiographs may be useful for identifying postmenopausal women with low bone mineral density (BMD) or osteoporosis. The purpose of this study was to evaluate whether these panoramic measurements are useful for identifying low BMD or osteoporosis in postmenopausal women younger than 65 years. We compared the diagnostic performances of panoramic measurements with those of the osteoporosis self-assessment tool (OST) for identifying women with low BMD (T-score of -2.0 or less at either the lumbar spine or the femoral neck) and osteoporosis (T score of -2.5 or less) in 158 healthy Japanese postmenopausal women aged 46 years to 64 years. Mandibular cortical shape (erosion) and width were evaluated on dental panoramic radiographs. Receiver operating characteristic curve analysis was used to determine the optimal cutoff thresholds for cortical width and OST index. The sensitivity and specificity, respectively, for identifying women with low BMD were 82.3% and 55.2% for OST index, 79.0% and 50.0% for cortical width, and 72.6% and 74.0% for cortical shape. The sensitivity and specificity, respectively, for identifying women with osteoporosis were 86.7% and 46.9% for OST index, 90.0% and 45.3% for cortical width, and 86.7% and 65.6% for cortical shape. Likelihood ratio for identifying women with low BMD was 13.90 for thin cortical width (<3.0 mm) and 10.84 for severely eroded cortex. That for identifying women with osteoporosis was 6.40 for thin cortical width and 7.11 for severely eroded cortex. Dentists may be able to refer postmenopausal women younger than 65 years for bone densitometry on the basis of incidental findings on dental panoramic radiographs.