Dual energy X-ray absorptiometry (DXA) has been well established in both clinical and research settings for measurement of bone mineral density (BMD), and is becoming more widely utilized for assessment of body composition. Reliability and validity are essential factors in both applications of this technique; however, neither have been confirmed for the QDR-4500A DXA at Virginia Tech. Therefore, measurements of the whole body (WB), lumbar spine (LS), total proximal femur (TPF) and total forearm (TF) were made in a group of young-adult males and females at two time-points, 5-7 days apart. Significant differences were not found in BMD (g/cm2) at these body sites with repeated measurements by DXA. Furthermore, measures of percent body fat (%BF), lean body mass (LBM), and fat mass (FM) by DXA were reliable. Validity of %BF by DXA was assessed from comparison to single-frequency bioelectrical impedance analysis (BIA). Significant differences were not found in measures of %BF by DXA and BIA. A second study investigated the reliability and validity of the QDR-4500A DXA in measurements of distal tibia (DT) BMD. Significant differences were not found between repeated measurements. Validity was established by a significant correlation between WB BMD and DT BMD. A third study examined the influence of navel jewelry on the accuracy of LS DXA measurements. Repeated measurements with a spine phantom revealed that both a navel ring and a barbell produced significantly greater measures of LS BMD compared to the spine phantom alone. Manual correction of navel jewelry did not eliminate BMD inaccuracies. Data from these studies confirmed that the QDR-4500A DXA at Virginia Tech was a reliable and valid device in measurement of WB, LS, TPF, TF and DT BMD, as well as %BF, LBM, and FM. In addition, effects of navel jewelry on LS BMD have been recognized. Further studies investigating the reliability and validity of DT BMD measures as well as effects of different types, gauges, and shapes of body jewelry on BMD measures in human subjects are warranted. / Master of Science