BACKGROUND: The ulnar artery is rarely selected for coronary angiography or percutaneous coronary intervention despite the expanding use of the transradial approach. We aimed to establish noninferiority of a default transulnar relative to transradial approach in terms of feasibility and safety.METHODS AND RESULTS: This was a prospective, randomized, multicenter, parallel-group study involving 902 patients at 5 sites eligible to undergo diagnostic coronary angiography and percutaneous coronary intervention. Patients were randomized in a 1:1 ratio to either transradial approach (reference intervention) or transulnar approach (experimental intervention) regardless of the Allen test results. The primary end point was a composite of cross-over to another arterial access, major adverse cardiovascular events, and major vascular events of the arm at 60 days. The study was prematurely terminated after the first interim analysis because of inferiority of the transulnar approach. Although the difference in the primary end point became inconclusive after adjustment for operator clustering (24.30%; 99.99% confidence interval [CI], -7.98% to 56.58%; P=0.03 at α=0.0001), need for cross-over in the transulnar group remained inferior to transradial access site with a difference of 26.34% (95% CI, 11.96%-40.69%; P=0.004).CONCLUSIONS: As a result of higher cross-over rates, a first-line transulnar strategy was proven inferior to the transradial approach for coronary procedures. At present, the transulnar route should not be regarded as an acceptable alternative to the transradial access site.