OBJECTIVE To evaluate the effects of heart rate and heart rate variability on image quality, patient dose and diagnostic accuracy of 320-detector row CT. METHODS 94 patients were prospectively enrolled. Heart rate was defined as the mean value of different intervals elapsing between two consecutive R waves in an electrocardiogram (R-R intervals) and the heart rate variability was calculated as the standard deviation from the average heart rate. The image quality was evaluated by four grades, according to motion artefacts ("step artefacts" and "blurring artefacts"). The diagnostic accuracy was analysed in 43 patients who were scheduled for invasive coronary angiography (ICA). The coeffects of heart rate and heart rate variability on image quality, radiation dose and diagnostic accuracy were evaluated by multivariate regression. RESULTS The mean image quality score was 1.2 ± 0.5 and the mean effective dose was 14.8 ± 9.8 mSv. The results showed that heart rate (74.0 ± 11.2 beats per minute) was the single factor influencing image quality (p<0.001) and radiation dose (p<0.001), while heart rate variability (3.7 ± 4.6) had no significant effect on them (p=0.16 and p=0.47, respectively). For 43 patients who underwent ICA, heart rate and heart rate variability showed no influence on the accuracy (p=0.17 and p=0.12, respectively). Overall sensitivity was 97.4% (37/38), specificity was 99.4% (351/353), positive predictive value was 94.9% (37/39) and negative predictive value was 99.7% (351/352). CONCLUSIONS 320-detector row CT, with improved longitudinal coverage of detector, resolves step artefact and high patient dose caused by irregular heart rate. However, it is still recommended to control heart rate to a lower level to eliminate blurring artefact and radiation dose.