Percutaneous radial artery cannulation is widely used for direct continuous arterial blood pressure measurement and sampling of arterial blood. We compared the success rate of arterial catheter placement in patients undergoing aortocoronary bypass operations using the direct and the modified Seldinger techniques. The effects of gender and quality of the pulse were also investigated. Our study group consisted of 42 female and 96 male patients. The overall success rate with the guide wire was 82% compared with that of the direct method of 65% (P = 0.02). The success rate for arterial cannulation in male patients was high whether direct cannulation or a guide wire was used. In female patients, on the other hand, the failure rate with the direct technique was high (57%) and was significantly lower (14%) when the guide wire was used (P < 0.001). We conclude that the success rate for cannulation is high in male patients, and patients with a bounding pulse regardless of the use of the direct or guide-wire techniques. The guide wire is recommended as the initial technique for cannulating the radial artery of female patients. In patients with a thready pulse, no significant advantage could be obtained by using a guide wire, but in salvaging an arterial line the guide wire is efficacious.