Low bone mass is the most important and individually objective predictor of osteoporotic fragility fracture risk. The challenge to the clinicians who care for patients with low bone mass is to identify those who are asymptomatic and nonfractured by using a bone mass measurement technique to make a diagnosis and decide on the level of intervention. All bone mass measurement techniques have value and limitations, necessitating careful individual clinical interpretation to avoid a misdiagnosis of osteoporosis or an erroneous interpretation of serial changes in bone mass. The inverse relationship between low bone mass and increasing fracture risk needs to be carefully interpreted because this relationship has only been determined in patients with a mean age of 65 years. It is not the same in younger, perimenopausal patients. Increasing age is a strong, independent risk factor for fragility fracture. In patients older than 70 years the curves relating fracture risk to low bone mass density become exponential when more than 3.0 standard deviations below peak adult bone mass. The assessment of this relationship is a professional activity and needs clinical input from appropriately educated physicians.