Dual-energy X-ray absorptiometry (DXA) is actually considered as one of the most appropriate techniques for measuring bone mineral content (BMC) and bone mineral density (BMD). An anthropomorphic phantom and a 25-year-old girl were repeatedly measured, 160 times and 50 times respectively, over an 18-month period to investigate performance in vitro and in vivo of a commercial DXA equipment (HOLOGIC QDR 1000). DXA is a highly accurate technique, the BMC and BMD determinations only overestimated the exact value of the phantom by 0.20% and 0.51% respectively. In vivo long-term (18 months) reproducibility of BMD of the spine is characterized by an interassay coefficient of variation (CVt) of 0.8% while, for the different regions of interest of the hip, BMD CVt varies from 1.1% (total zone) to 5.3% (Ward's triangle). In the subject tested, BMD sensitivity for changes of 2.2% at the lumbar spine and 3% at the hip were recorded.