Radial artery function was studied in 114 consecutive patients by arteriography, Doppler ultrasound flow detection, and physical examination following prolonged (1-10 days) percutaneous cannulation with a single type of 20-gauge catheter. Cannulations lasting 1-3 days produced 11% arterial occlusion, whereas those lasting 4-10 days induced 29% incidence of occlusion (p less than 0.05). In addition, a significantly higher incidence of cannula dysfunction (38% versus 18%, p less than 0.05) and thrombus formation (0-3+ scale) was also observed in the group cannulated 4-10 days as compared with the group of shorter duration. The risk of vascular complications arising from percutaneous radial artery cannulation with 20-gauge catheters increases markedly after 3 days.