There are anatomical differences between right and left radial artery approaches for coronary catheterization that could influence application of the technique. We present the results of a randomized study that compared the effectiveness of the two approaches and identified factors associated with failure of the procedure. The study involved 351 consecutive patients: a left radial approach was used in 180, and a right radial approach, in 171. The procedure could not be completed using the initial approach selected in 15 patients (11 right radial vs. 4 left radial; P=.007). Use of a right radial approach, lack of catheterization experience, patient age >70 years, and the absence of hypertension were found to be independently associated with prolonged fluoroscopy duration and failure using the initial approach. Use of the right radial approach in patients aged over 70 years was associated with a 6-fold increase in the risk of an adverse event. Consequently, use of the right radial approach should be avoided in patients aged over 70 years when trainee practitioners are on the learning curve.