OBJECTIVE To determine if either wire-guided-catheters (WGC) or nonwire-guided catheters (non-WGCs) are associated with a higher rate of successful arterial cannulation overall or when evaluated by a variety of patient and operator characteristics. METHODS Prospective clinical trial in a ten-bed adult medical-surgical ICU in a 500-bed university hospital. METHODS Adults requiring arterial cannulation for hemodynamic monitoring or frequent blood sampling. A total of 116 attempted arterial cannulations were recorded, and 112 in 67 patients were acceptable for statistical analysis. RESULTS Overall, no difference in success rates could be demonstrated between WGC and non-WGC. WGCs were associated with a higher success rate in patients with pulses characterized as absent or weak (78% vs 37%; p=0.01). WGCs also had a greater success rate than non-WGCs for more experienced operators, whether defined by seniority (83% vs 44%; p=0.02) or by experience at arterial catheterization (81% vs 48%; p=0.02). CONCLUSIONS WGCs were not associated with better success rates overall than were non-WGCs, although there were higher success rates in several subsets of patients and/or operators. Because of their significantly higher costs, WGCs cannot be recommended for routine use for arterial catheterization. In cases where successful catheterization is essential, however, there are circumstances under which they may be the preferred equipment, depending on patient characteristics and operator experience.