Background: In recent years, the radial artery (RA) has become an alternative vascular access site for percutaneous coronary procedures, and the ulnar artery (UA) is another possibility. The objective of this study was to investigate the anatomy of the forearm arteries with ultrasound (US) and to evaluate the effect of the anatomy of the right RA (RRA) on the outcomes of transradial coronary procedures. Methods and Results: The 638 patients undergoing transradial coronary procedures were examined with US for measurement of the diameters of the forearm arteries and determination of their anatomical abnormalities before the procedures. The next day the incidence of RA occlusion was recorded. The diameters of the radial and ulnar arteries were similar (P>0.05). The procedure time was longer in patients with anatomical abnormalities (P<0.05) and whose RRA had a diameter <2 mm (P<0.05). The incidence of procedure failure, and of RA occlusion one day after the procedure was also higher in patients with an anatomical abnormality of the RRA (P<0.01 and P<0.05, respectively) and whose RRA diameter was <2 mm (P<0.05 and P<0.05, respectively). Conclusions: The diameters of the forearm arteries of Chinese people are similar. The small diameter and anatomical abnormalities of the RRA could result in longer procedure time, more incidence of procedure failure and RA occlusion. (Circ J 2010; 74: 686-692)