Computed tomography (CT) coronary angiography has been widely used since the introduction of 64-slice scanners and dual-source CT technology, but high radiation doses have been reported. Prospective ECG-gating using a 'step-and-shoot' axial scanning protocol has been shown to reduce radiation exposure effectively while maintaining diagnostic accuracy. 256-slice scanners with 80 mm detector coverage have been currently introduced into practice, but their impact on radiation exposure has not been adequately studied. The aim of this study was to assess radiation doses associated with CT coronary angiography using a 256-slice CT scanner. Radiation doses were estimated for 25 patients scanned with either prospective or retrospective ECG-gating. Image quality was assessed objectively in terms of mean CT attenuation at selected regions of interest on axial coronary images and subjectively by coronary segment quality scoring. It was found that radiation doses associated with prospective ECG-gating were significantly lower than retrospective ECG-gating (3.2 +/- 0.6 mSv versus 13.4 +/- 2.7 mSv). Consequently, the radiogenic fatal cancer risk for the patient is much lower with prospective gating (0.0176% versus 0.0737%). No statistically significant differences in image quality were observed between the two scanning protocols for both objective and subjective quality assessments. Therefore, prospective ECG-gating using a 'step-and-shoot' protocol that covers the cardiac anatomy in two axial acquisitions effectively reduces radiation doses in 256-slice CT coronary angiography without compromising image quality.