OBJECTIVE To investigate the diagnostic accuracy and limitations of 16-slice multidetector computed tomography (MDCT) in the detection of significant ( > 50%) stenosis of coronary artery bypass grafts (CABG) and native coronary arteries. METHODS One hundred two patients with 236 grafts were investigated by 16-slice MDCT. Native coronary arteries were also investigated. The image quality was assessed in terms of artifact, and the evaluable segments were screened for the presence of occlusion and significant ( > 50%) stenosis. MDCT results were compared with conventional coronary angiography. RESULTS The evaluability of MDCT was 90.4% for CABG and 71.2% for native coronary arteries. The most frequent causes of nonevaluable segments were motion artifact in venous grafts, metallic clip artifact in arterial grafts, and severe calcification in native coronary arteries. MDCT correctly diagnosed all of the 46 occluded grafts. The sensitivity, specificity, and the positive and negative predictive value of MDCT for the detection of significant CABG stenoses were 91.4%, 98.5%, 84.2%, and 99.2%, respectively. Including nonevaluable segments in the analysis, overall sensitivity was 84.2%. For the evaluation of native coronary arteries, MDCT had a sensitivity of 82.1% and a specificity of 75.3%, but evaluability was only 71.2%, resulting in overall sensitivity of 62.1%. CONCLUSIONS Use of 16-slice MDCT angiography allows very accurate evaluation of CABG patency and has high diagnostic accuracy in detecting graft stenoses. But evaluation of native coronary artery stenosis is limited, particularly in patients with advanced coronary artery disease with severe calcification.