Low bone density is a condition where the tissue inside your bones begins to thin, or becomes less dense. There are two forms of this condition: osteopenia (mild low bone density) and osteoporosis (severe low bone density). Osteoporosis is a systemic skeletal disease characterized by decreasing bone mass and microarchitectural deterioration of bone tissue, with consequent increases in bone fragility and susceptibility to fracture. In addition to fractures, the clinical complications of osteoporosis include disability and chronic pain. Approximately 52 million people in the United States are affected by osteoporosis or low bone density. It is especially common in postmenopausal women, but one in five men will experience an osteoporosis-related fracture at some point in his lifetime. The economic burden of osteoporosis is large and growing: the most recent estimate of U.S. annual costs due to fractures alone have been nearly $20 billion. A recent projection of the burden and costs of incident osteoporosis-related fractures in the United States from 2005 to 2025 estimates more than 2 million fractures in 2010, with direct medical costs of more than $18 billion (more than 25 percent attributable to men). Although the bulk of these costs are incurred by individuals 65 and older, direct costs and productivity loss among working women under 65 are considerable. This summary explains how low bone density can increase the risk of breaking a bone and what research has found about different treatments to lower the risk. It can help you talk with your doctor about which treatment might be right for you.