Background— To quantify the changes in arterial dimensions after the acute changes in pressure associated with percutaneous coronary intervention (PCI). Methods and Results— Forty-eight patients with one angiographically moderate-to-severe stenosis were included in the study. The pressure proximal and distal to the stenosis and the arterial diameter proximal and distal to the stenosis were measured at baseline, after intracoronary nitrates, and after stent PCI. In addition, in 8 patients distal pressure and coronary diameter were assessed while graded, controlled stenoses were created in the stented segment by progressive inflation of a balloon catheter. The mean diameter of the proximal coronary segment was 2.75±0.08 mm, 2.92±0.08 mm (+7.4%), and 3.10±0.07 mm (+14.7%) at baseline, after nitrates and after PCI, respectively ( P <0.001). The mean diameter of the distal coronary segment was 2.07±0.09 mm, 2.23±0.09 mm (+9.7%), and 2.5±0.07 mm (+28.4%) at baseline, after nitrates and after PCI, respectively ( P <0.001). The increase in distal diameter correlated significantly with the increase in distal pressure after PCI ( r 2 =0.57; P <0.001). When graded stenoses were created, a decrease in diameter of 18±4% was observed with a pressure drop of 43±5 mm Hg. Conclusions— The diameter of coronary arteries markedly varies with their distending pressure. After re-establishment of a normal distending pressure by stenting of severe coronary stenoses, a proportional increase in vessel diameter is observed. This should be taken into account when choosing the stent diameter and is an argument to discourage direct stenting.