Catheter malposition after subclavian venous catheterization (SVC) is not uncommon and can lead to serious complications. This study hypothesized that the left access is superior to the right access in terms of catheter malposition after ultrasound-guided infraclavicular SVC due to the asymmetry of the bilateral brachiocephalic veins. Parallel-armed randomized controlled trial. A tertiary referral hospital in Korea. Patients 20-79 years old who were scheduled to undergo SVC under general anesthesia. Patients were randomly assigned to either the left ( n = 224) or right ( n = 225) SVC group. The primary outcome measure was the overall catheter malposition rate. The secondary outcome measures included catheter malposition rates into the ipsilateral internal jugular and contralateral brachiocephalic veins, other catheterization-related complications, and catheterization performance. The catheter malposition rate was lower (10 [4.5%] vs. 31 [13.8%], p = 0.001), especially in the ipsilateral internal jugular vein (9 [4.0%] vs. 24 [10.7%], p = 0.007), in the left SVC group than in the right SVC group. In the left SVC group, catheterization success rates on the first pass (88 [39.3%] vs. 65 [28.9%], p = 0.020) and first-catheterization attempt (198 [88.4%] vs. 181 [80.4%], p = 0.020) were higher whereas times for vein visualization (30 s [18-50] vs. 20 s [13-38], p < 0.001) and total catheterization (134 s [113-182] vs. 132 s [103-170], p = 0.034) were longer. There were no significant differences in other catheterization performance and catheterization-related complications between the two groups. These findings strengthen the rationale for choosing the left access over the right access for ultrasound-guided infraclavicular SVC.