Background: We sought to determine the utility of absence of coronary artery calcium (CAC) in excluding obstructive coronary artery disease (CAD) in patients who were referred to perform coronary CT angiography (CTA). Methods: We examined 2568 patients (30-69 years) who were referred to perform CTA with suspicion of CAD or for exclusion of CAD from Jan 2008 to Feb 2009. Patients with previous history of myocardial infarction, PCI or bypass surgery were excluded. Patients underwent CAC scoring with 64-slice MSCT. We analyzed sex and age of patients with absence of CAC and obstructive CAD detected on CTA. All patients were Korean. Results: There were 1385 men (mean 56.1 years) and 1183 women (mean 58.8 years). CAC was absent in 628 men (45.3%) and 736 women (62.2%). In the patients without CAC, noncalcified plaques were detected in 77 men (12.3%, age 38-69 years) and 53 women (7.2%, age 45-69 years). Obstructive CAD were found more in men (n=36, 5.7%, mean 52.0 years, range 38-71 years) than women (n=14, 1.9%, mean 56.7 years, range 48-68 years). The prevalence of obstructive CAD in patients without CAC was increased according to age in both men and women; 2 (2.5%) and 0 (0%) in 30-39 years, 8 (4.4%) and 2 (1.8%) in 40-49 years, 11 (4.8%) and 6 (1.9%) in 50-59 years, and 15 (10.8%) and 6 (2.2%) in 60-69 years. The prevalence of obstructive CAD in patients without CAC according to pretest probability in men and women was as follows; 8% and 5% in high probability, 5.8% and 1.8% in intermediate probability, 6% and 3.2% in low probability, and 0% and 0% in very low probability. Conclusion: Even though absence of calcium does not guarantee absence of obstructive CAD, in the absence of CAC in young women, obstructive CAD could be excluded with a very high predictability regardless of presence of symptoms.