Purpose: A cross-calibration equation is needed to compare bone mineral density measured by different dual X-ray absoptiometry systems. We performed this study to establish appropriate cross-calibration equations between two different dual X-ray absorptiometry systems. Materials and Methods: Bone mineral density of anterior-posterior lumbar spine (L2-4 level) and femoral neck were measured in 109 women (55¡¾11yr) using two different dual X-ray absorptiometry systems (Lunar EXPERT-XL) and Hologic QDR 4500-A). Bone mineral density values measured by two systems, including area, bone mass content, bone mineral density and percentile of young normals were compared and cross-calibration equations between two systems derived. Results: The bone mineral density values of 109 women measured by Lunar system were 0.958¡¾0.17 g/cm2 at L2-4 and 0.768¡¾0.131 g/cm2 at femur neck, which were significantly higher (13¡¾6% at L2-4 and 19¡¾7% at femur neck, p<0.001) than those (0.851¡¾0.144 g/cm2 at L2-4 and 0.649¡¾0.108 g/cm2 at femur neck) by Hologic system. Bone mineral content and percentile of young normals measured by Lunar system were also significantly higher than those by Hologic system (p<0.001), whereas there was no difference in area (p>0.05). There was a high correlation between bone mineral density values of L2-4 and femoral neck obtained with both dual X-ray absortiometry systems (r=0.96 and 0.95, respectively). Cross-calibration equations relating the bone mineral density were Lunar=1.1287¡¿Hologic-0.0027 for L2-4 and Lunar=l.1556¡¿Hologic+0.0l82 for femoral neck. Conclusion: We obtained cross-calibration equations of bone mineral density between Lunar EXPERT-XL and Hologic QDR 4500-A. These equations can be useful in comparing bone mineral density obtained by different dual X-ray absorptiometry systems.