OBJECTIVE: To evaluate the effects of heparinized and nonheparinized flush solutions on the patency of arterial pressure monitoring lines. SAMPLE: A total of 5139 patients from 198 participating sites comprised the sample for this large-scale, randomized clinical trial. METHODS: Patients were randomly assigned to heparinized and nonheparinized flush solution treatment groups. Data were collected at 4-hour intervals for up to 72 hours on presence of acceptable square waveform and arterial backflow. RESULTS: Arterial pressure monitoring lines maintained with heparinized flush solutions had a significantly greater probability of remaining patent over time than lines maintained with nonheparinized flush solutions. In addition to heparin, four other variables significantly influenced the probability of lines remaining patent: receiving other anticoagulants or thrombolytics, having a catheter longer than 2 inches, femoral insertion site and male gender. CONCLUSIONS: Chances for patency are greatest in men with long femoral lines who receive other anticoagulants or thrombolytics and heparinized flush solutions. Risks of nonpatency are greatest in women with short nonfemoral lines who do not receive other anticoagulants or thrombolytics and have nonheparinized flush solutions.