Radial arterial function was evaluated in 108 patients following 24 h of percutaneous cannulation with either 18- or 20-gauge cannulas. Arteriography, Doppler ultrasound examination and Allen''s test disclosed an 8% incidence of radial-artery occlusion following cannulation with 20-gauge cannulas, compared with a 34% incidence of occlusion with 18-gauge cannulas (P < .05). Vessels that occluded were significantly smaller in diameter than were patent vessels (mean 2.00 .+-. .09 mm SE vs. 2.22 .+-. .05 mm, P < .05). Occluded vessels contained significantly greater amounts of thrombotic material (0-3+) just prior to decannulation than those that remained patent (2.42 .+-. .13 vs. 1.20 .+-. .11, P < .001). The incidence of arterial occlusion increases linearly as the ratio of cannula outer diameter to vessel-lumen diameter increases.