BACKGROUND The association between hydraulic forces generated during contrast injection and the risk of coronary injury is poorly understood. We evaluated whether contrast injections increase intracoronary pressures beyond resting levels and estimated the risk of hydraulic propagation of coronary dissections. METHODS This prospective multicenter study included patients with non-flow-limiting coronaries. A continuous 60-second pressure recording was taken in 5 predetermined locations during contrast injections: distal, mid, and proximal vessel, catheter tip, and inside catheter. The primary endpoint was the change in intracoronary peak pressure between resting and injections in each location. RESULTS 269 pressure recordings (58 vessels, 52 patients) were analyzed. Injections led to small increase in peak pressure in the distal (mean difference [MD]: +4.5 mm Hg; 95% CI: 1.5-7.4), mid (MD: +4.1 mm Hg; 95% CI: 1.4-6.9), and proximal (MD: +5.1 mm Hg; 95% CI: 2.5-7.7) vessel locations, and much higher increases at the catheter tip (MD: +11.7 mm Hg; 95% CI: 5.8-17.7) and inside catheter (MD: +77.5 mm Hg; 95% CI: 64.5-90.4). Compared with the distal vessel, pressure changes were only significant at the catheter tip (+10 mm Hg; P < 0.01) and inside catheter (+79.1 mm Hg; P<0.01). CONCLUSIONS Contrast injections lead to negligible changes in intracoronary pressures beyond the catheter tip. Although injections should be avoided when dissections are close to the catheter, it is unlikely that they would cause injuries beyond the catheter tip.