Background: Evidence is accumulating that RANKL (receptor activator nuclear factor kB ligand)/RANK/OPG(osteoprotegerin) system, molecules of the tumor necrosis factor –related family, may be expressed, regulated, and function in vascular physiology and pathology in a unique way. The aim of this study was to investigate the role of RANKL and OPG in atherosclerosis by assessing the relationship between plaque burden in volumetric intravascular ultrasound(IVUS) analysis and plasma OPG/RANKL level. Methods: In 38 patients(mean age: 61짹10yrs, M:F=22:16) who had single lesion on the proximal or mid left anterior descending coronary artery(LAD), we performed IVUS evaluation using automatic pullback device. Cross sectional images at 1 mm interval were analyzed. The atheroma areas were calculated with EEM(external elastic membrane)_area minus lumen area(Lumen_area) measured by manual planimetry. Total atheroma volume(TAV) was calculated as the sum of atheroma area, normalized total atheroma volume(normalizedTAV) as TAV/(the number slices in pullback)��� median number of image slice in study population, and percent atheroma volume(PAV) as sum of atheroma area/sum of EEM _area���100. Results: 1. Plasma OPG levels were not correlated with TAVs, normalizedTAVs, or PAVs. 2. Plasma RANKL levels were correlated with TAVs(r=0.32, p<0.05), normalized TAVs(r=0.37, p<0.05) and PAVs(r=0.33, p<0.05). 3. The OPG/RANKL ratios were negatively correlated with PAVs(r=-0.32, p<0.05). Conclusion: The plasma RANKL level and OPG/RANKL ratio is associated the severity of atherosclerotic burden in patients with single lesion on proximal or mid LAD. Further investigation is needed to reveal if and how RANKL plays an active role on atherogenesis.