ObjectivesWe aimed to compare the success and complication rates of radial artery catheterizations between ultrasound guidance and conventional palpation technique in obese patients by anesthesia residents with similar levels of experience in both methods. Another objective was to measure the skin-to-artery distance of radial, brachial, and dorsalis pedis arteries using ultrasound with standardized anatomical landmarks. DesignProspective, randomized controlled trial SettingSingle tertiary center ParticipantsEighty adults with a body mass index (BMI) ≥ 30 kg·m-2 InterventionsUltrasound guidance or conventional palpation method Measurements and Main ResultsThe primary outcome was the first-attempt success rate of arterial catheterizations. The skin-to-artery distance of the radial artery in the BMI groups of 40–49 and ≥ 50 kg/m2 was significantly greater than that of the BMI group of 30–39 kg·m-2, mean difference: 1.0 mm [95% confidence interval (CI): 0.4 – 1.7, p=0.0029] for BMI 40–49 vs. 30–39 kg/m2 and 1.5 mm (95% CI: 0.6 – 2.4, p=0.0015) for ≥ 50 vs. 30–39 kg·m-2. Similar findings were observed for the brachial artery. BMI was inversely associated with first-attempt success rates (p=0.0145) and positively with time to successful catheterization (p=0.0271). The first-attempt success and vascular complication rates of catheterization did not significantly differ between the ultrasound guidance group (65.0%; 52.5%) and the conventional palpation group (70.0%, p=0.6331; 57.5%, p=0.6531). ConclusionOur results do not support routine uses of ultrasonography during radial arterial catheterizations for obese adults when junior practitioners performed the procedure.