PURPOSE: To compare, through enalysis of the coronary artery calcium (CAC) score and the risk factors for atherosclerosis, the characteristics of acute coronary syndrone between an asymptomatic high-risk group of atherosclerosis patients and a chronic coronary arterial obstructive disease(CAOD) group. MATERIALS AND METHODS: The CAC scores of an asymptomatic high-risk group of atherosclerosis patients (group I, n=284), a chronic CAOD group (group II, n=39) and an acute coronary syndrome group (group III, n=21) were measured by electron beam tomography. Forty-seven patients with CAOD from groups II and III underwent coronary angiography, and we scrutinized age, sex, and risk factors including diabetes mellitus, hypertension, obesity, smoking, hypercholesterolemia and low high-density lipoproteinemia. The numbers of stenotic coronary arterial branches and degree of stenosis revealed by coronary angiography were also recorded. We determined the differences between the three groups in terms of CAC score and the risk factors, the relationship between CAC score and risk factors, and the characteristic features of each type of CAOD group. RESULTS: The mean CAC score of group III (135.1) was not statistically different from that of group I (135.7) or group II (365.8). Among patients aged below 50, the mean CAC score of group III (127.4) was significantly higher than that of group I (6.2), (p=0.006). The mean CAC score at the sixth decade was also significantly different between group I (81.5) and group II (266.9). The mean age of group III (54.2 years) was significantly lower than that of group I (58.1 years) (p=0.047) and of group II (60.1) (p=0.022). There was significant correlation between the number of stenotic coronary arterial branches and log(CAC+1) (p<.01). The square root of the CAC score and the maximal degree of stenosis was also well correlated (p<.01). There was no difference in the mean number of risk factors among the three groups, though the incidence of smoking in group III was significantly higher than in groups I and II. Multiple regression analysis showed that the CAC score was related to age, diabetes mellitus and hypertension in group I, diabetes mellitus only in group II, but no particular factor in group III. CONCLUSION: The CAC score of the acute coronary syndrome group tended to be lower than that of the chronic CAOD group. It appears to be difficult to predict acute coronary syndrome on the basis of CAC alone. Compared with the asymptomatic high-risk group of atherosclerosis patients, the acute coronary syndrome group, whose members are younger and have a higher incidence of smoking, has a relatively high CAC score.