T he WHO Fracture Risk Assessment Tool (FRAX) calculator is the most thoroughly studied and widely used tool of fracture risk assessment. (1) FRAX is now available in 39 countries and is increasingly being used as a guide for clinical decision-making. The specific objective of developing FRAX was to aid clinicians in identifying the most appropriate patients to receive pharmacologic therapy to reduce fracture risk. Prior to FRAX, treatment decisions were based primarily on bone mineral density (BMD) values with the threshold at which treatment was recommended being modified crudely by the simple presence of absence of other clinical risk factors. For example, the 1998 National Osteoporosis Foundation (NOF) guidelines suggested treating all postmenopausal women with a BMD T-score of À2 or lower and all women with a T-score of À1.5 or lower who had one of several risk factors, including being thin, having a fragility fracture as an adult or a family history of fragility fracture. (2) These guidelines identified for therapy almost all patients at high risk for fracture (sensitivity of the approach was good) but enfranchised therapy for many younger postmenopausal women with low bone density but who were at modest or even low fracture risk (poor specificity). Understanding this, clinicians asked themselves ''Which patients who do not have osteoporosis should be treated?'' FRAX was specifically developed to answer that question.After the 1998 NOF guidelines were released, we recognized that individuals could be stratified into gradations of fracture risk more effectively if BMD was combined with other independent clinical risk factors for fracture than could be accomplished by using individual risk factors for such as age, BMD, or fracture history alone. (3) The FRAX tool was derived by evaluating the relationships between BMD, clinical risk factors, and fracture risk using individual patient data comprising almost 250,000 patient-years of observation in nine large observational studies performed in different parts of the world. (1) Subjects in those studies were generally healthy older adults who were not on osteoporosis treatment. In the United States and Europe, FRAX accurately predicts the 10-year probability of major osteoporotic fracture and hip fracture in patients with characteristics like the study subjects. (4,5) Importantly, because FRAX was designed to identify patients for whom treatment would be beneficial, analyses of clinical trials with different classes of osteoporosis treatments have documented that this is true. (6)(7)(8)(9) Since its availability in 2008, FRAX estimates of fracture probability have been incorporated, albeit in various ways, into clinical guidelines of several national societies including the NOF. (10)(11)(12) The fracture risk thresholds at which treatment is recommended in the NOF guidelines were based on clinical and health economic considerations. (13) Despite the scientific basis of FRAX and its strengths, several limitations are recognized. FRAX treats all risk factors as ca...