BACKGROUND: Intravascular ultrasound (IVUS) guidance during percutaneous coronary intervention (PCI) has been associated with increased stent optimization and reduced adverse events among patients with complex coronary-artery lesions, but adoption of this strategy in Western countries remains low. Although practice guidelines recommend intracoronary imaging for anatomically complex lesions, evidence from current European practice is limited. METHODS: In this investigator-initiated, international, open-label, randomized, controlled trial, we assigned patients undergoing complex PCI to either IVUS-guided PCI, performed with the use of prespecified stent-optimization criteria, or angiography-guided PCI. The primary end point was target-vessel failure, defined as a composite of death from cardiac causes, target-vessel myocardial infarction, or clinically indicated target-vessel revascularization. RESULTS: Of the 2020 patients who underwent randomization, 1010 in the IVUS-guided PCI group and 1009 in the angiography-guided PCI group were included in the primary analysis. The mean age of the patients was 69 years, 79.4% were men, and 27.4% presented with an acute coronary syndrome. The mean procedure duration was 88.8 minutes with IVUS-guided PCI and 66.2 minutes with angiography-guided PCI. Dilation with balloon angioplasty after stent implantation was performed in 91.3% of the IVUS-guided PCI procedures and in 84.5% of the angiography-guided PCI procedures. At a median follow-up of 19.0 months (interquartile range, 15.2 to 23.4), target-vessel failure had occurred in 140 patients (13.9%) in the IVUS-guided PCI group and in 112 patients (11.1%) in the angiography-guided PCI group (hazard ratio, 1.25; 95% confidence interval, 0.97 to 1.60; P = 0.08). Procedural complications occurred in 11.3% of the IVUS-guided PCI procedures and in 10.2% of the angiography-guided PCI procedures. The frequency of adverse events appeared to be similar in the two groups. CONCLUSIONS: Among patients undergoing complex high-risk PCI, a strategy of routine IVUS-guided PCI performed with the use of prespecified stent-optimization criteria was not associated with a lower risk of target-vessel failure than angiography-guided PCI alone. (Funded by Boston Scientific; IVUS-CHIP ClinicalTrials.gov number, NCT04854070.).