To test the feasibility of using guiding catheters equal to or greater than 7 Fr in transradial coronary intervention (TRI), we measured the inner diameter of the radial artery (RA) and its flow using two-dimensional ultrasound and Doppler examinations before and after TRI in 250 Japanese patients. The incidence of severe flow reduction of the RA after TRI was 6.8% on average (7.2%, 4.7%, and 8.3% in patients with 6, 7, and 8 Fr sheaths used, respectively), which increased from 4.0% to 13.0% (P=0.0113) if the ratio of the RA inner diameter/sheath outer diameter was less than 1.0. The frequency of this ratio (> or = 1.0) for 7 and 8 Fr sheaths was 71.5% and 44.9% in male patients and 40.3 and 24.0% in female patients, respectively. We conclude that the use of guiding catheters equal to or greater than 7 Fr in the radial artery is feasible in selected patients.