The majority of coronary artery blood flow occurs in diastole; however, systolic epicardial coronary artery expansion has been described. With the advent of intravascular ultrasound, precise measurements of arterial structures with excellent spacial and temporal resolution are now readily available. However, the effect of dynamic expansion of the coronary arteries on routine intravascular ultrasound measurements has not been assessed. The purpose of this study was to determine in vivo the presence, timing, and extent of dynamic changes in the coronary arteries and saphenous vein grafts and to assess their implications for intravascular ultrasound measurements. Intravascular ultrasound images were obtained with simultaneous electrocardiographic monitoring in 202 coronary artery and 50 saphenous vein graft sites in 32 patients with varying plaque burden and morphologic features. Arterial, luminal, and plaque area were measured at end-diastole and early, mid-, and end-systole. Coronary luminal diameter increased 2.1%; luminal area increased 8.1%; arterial area increased 3.7%; and plaque area decreased 4.9% during mid and late systole (p < 0.01). There was no detectable cyclic change in saphenous vein graft dimensions. In coronary arteries there was significant systolic expansion of the artery and lumen and systolic thinning of the plaque. The magnitude of dynamic luminal area change was greater than the variability in measurement and thus warrants gating to the cardiac cycle. The lack of dynamic change in saphenous vein grafts and the relatively small dynamic change in luminal diameter and arterial and plaque areas suggest nominal utility in gating these measurements to the cardiac cycle.