Purpose Previous studies have shown a correlation between axial pulmonary trunk diameter (PTD) on chest computed tomography (CT) and pulmonary artery pressure. However, it is not known whether the PTD slices measured on chest CT have been recorded during the systolic or diastolic phase. The aim of this study was to demonstrate the variations in PTD during the cardiac cycle by measuring coronary CT angiography (CCTA) images. Methods A retrospective analysis was made of 101 patients who underwent CCTA for coronary artery disease assessment. CCTA images were reconstructed during a full cardiac cycle and measurements were taken of the systolic and diastolic PTD and ascending aorta diameter (AAD) from the same slice by two independent observers. Results Inter-observer agreement was excellent (intraclass correlation coefficient = 0.99) for all CT measurements. The mean systolic PTD of all patients was 26.3 +/- 3.6 mm and the mean diastolic PTD was 22.8 +/- 3.2 mm (p < 0.001). The mean difference between systole and diastole was found to be 3.5 +/- 1.2 mm for PTD, 1.2 +/- 0.7 mm for AAD, and 0.1 +/- 0.04 for the PTD/AAD ratio (p values < 0.001). There was no statistical significance of PTD variations according to gender, age, height, weight, body mass index, and body surface area. Conclusion When an increased PTD is detected in a chest CT compared to normal limits or a previous CT scan, this may be the result of the variation in PTD due to the cardiac cycle.