BACKGROUND: Data on the influence of flush methods, blood-sampling methods, and site location on the patency of radial arterial catheters used for pressure monitoring are sparse. OBJECTIVES: To determine the effects of flush and blood-sampling methods, insertion site, and sex of patients on catheter patency. METHODS: In a randomized trial, 174 patients requiring radial arterial pressure monitoring were assigned to 4 groups: fast flush as needed and nonwaste blood sampling; fast flush as needed and waste blood sampling; fast flush every 4 hours and waste blood sampling; and fast flush every 4 hours and nonwaste blood sampling. All site locations were evaluated for patency, and all monitoring systems were maintained with isotonic sodium chloride solution. RESULTS: Nonpatent catheters were 4.23 times more likely in patients with insertion sites 3 cm or higher above the bend of the wrist than in patients with lower sites (P = .01). Duration of patency did not differ between catheters maintained with fast flush every 4 hours and those flushed as needed or between catheters according to the method of blood sampling. Women were 3.05 times more likely than men to have nonpatent catheters (P = .02). With insertion sites 3 cm or higher above the radiocarpal joint, nonpatency was 7.3 times more likely in women than in men (P < .001). CONCLUSIONS: Insertion sites closest to the bend of the wrist increase chances of maintaining patency. Catheters can be maintained with as-needed flushes, and either waste or nonwaste blood sampling can be used.