Background and Aim: The impaired coronary flow reserve (CFR) is one of the early manifestations of coronary artery disease (CAD). Osteoprotegerin (OPG) has been suggested to have an important pathophysiologic role in CAD. The aim of this study was to elucidate the relation between the serum OPG level and CFR. Methods: In 99 subjects (mean 56짹11yrs,M:F=55:44) with chest pain and normal coronary angiogram, CFR was measured using adenosine(140g/min/kg) with transthoracic Doppler echocardiography(TTE) in distal LAD coronary artery. The serum OPG levels were measured by enzyme-linked immunoassay. A low CFR level was defined as less than 2.0 and all subjects were classified into 4 subgroups [normotensives with CFR 2.0(GroupI,49[plusminus]11yrs, M:F=19:11, CFR = 3.17[plusminus]0.62), normotensives with CFR竊�2.0 (Group II, 54[plusminus]9 yrs, M:F= 8:6, CFR= 1.70[plusminus]0.36), hypertensives with CFR 2.0(Group III, 60[plusminus]8 yrs, M:F = 19: 21, CFR = 3.10[plusminus]0.67), hypertensives with CFR竊�2.0 (Group IV, 60[plusminus]11 yrs, M:F =9:6, CFR = 1.75[plusminus]0.37)]. Results:1. The serum OPG level was 948[plusminus]193 pg/ml in Group I, 1047[plusminus]222 pg/ml in Group II, 1526[plusminus] 512 pg/ml in Group III and 1729[plusminus]596 pg/ml in Group IV.(p<0.05 vs Group I, respectively) 2. The serum OPG level was significantly higher in subjects with the impaired CFR either hypertensives or normotensives(p<0.05, respectively). 3. There was a close relation between CFR and the serum OPG level in Group III (R=0.453, p<0.05).4. There was a close relation between age and the serum OPG level (R2=0.431, p<0.05). Conclusions: Serum OPG level is elevated in patients with impaired CFR and normal coronary angiograms. The factors that are associated with the serum OPG and the relation between the serum OPG level and CFR should be further evaluated