Although catheterization of the subclavian vein is a common procedure, the risk factors for complications and failures, with the exception of the physician's experience, are poorly understood. Ultrasonography has been recommended to help guide the placement of central venous catheters. We conducted a prospective randomized trial of ultrasound-guided location of the subclavian vein as compared with standard insertion procedures. In the group of patients undergoing catheterization with ultrasound guidance, the site of the insertion was marked before the catheterization attempt; real-time ultrasound guidance was not used. The 821 eligible patients (411 in the ultrasound group and 410 in the control group) underwent catheterization in a single procedure suite under controlled nonemergency conditions, in most cases for the administration of chemotherapy. Ultrasound guidance had no effect on the rate of complications or failures of subclavian-vein catheterization (risk ratio for complications, 1.00; 95 percent confidence interval, 0.66 to 1.52; risk ratio for failures, 1.04; 95 percent confidence interval, 0.72 to 1.50). In multivariate analyses, prior major surgery in the region (P = 0.002), a body-mass index (the weight in kilograms divided by the square of the height in meters) higher than 30 or lower than 20 (P = 0.009), and previous catheterization (P = 0.043) were associated with failed attempts. Complications were also associated with failed attempts: 52 of the 721 patients (7.2 percent) in whom catheterization was successful had complications, as compared with 28 of the 100 patients (28 percent) in whom physicians were unable to place catheters. The number of needle passes was strongly associated with the rates of failure and complications. The complication rate rose from 4.3 percent with one pass to 24.0 percent with more than two passes. Ultrasound guidance of subclavian-vein catheterization, as used in this study, was not beneficial. In patients at highest risk for complications and failures, catheterization should be attempted by the most experienced physicians available.