Central venous catheterization (CVC) is vital for anesthetic management in pediatric patients undergoing major surgery [1]. The internal jugular vein (IJV), subclavian vein (SCV), and femoral vein are the most commonly used sites for CVC. Unfortunately, the failure rate and incidence of complications are much higher in pediatric patients due to the relatively small size of the vessels [2]. Experience is essential in order to successfully perform CVC in pediatric patients without causing complications. Other significant factors include the selection of an appropriate vein and a catheter of a suitable size. The IJV is relatively larger and easier to access than the SCV, and has been preferred for CVC in pediatric patients by many physicians. Yet, the IJV is easily collapsed by the ultrasound (US) probe or by needle pressure, making CVC challenging even though the vein may be well observed via US. In contrast, the SCV is not easily collapsed due to the presence of the clavicle and can be more appropriate when long-term catheter placement is required [3]. Although there are several studies regarding the size of the IJV in pediatric patients [1,2], few studies have been performed related to the size of the SCV. We attempted to find the most suitable variable, such as age, weight, or height, that could predict the size of the SCV in pediatric patients by measuring the size of the SCV in a group of these patients using ultrasound. Thirty-eight pediatric patients from 1 month to 10 years of age, who underwent an elective surgery were included in this study. Patients included in the study were scheduled for minor operations: inguinal hernia repair, hydrocelectomy, circumcision, and upper and lower extremity surgeries. Patients with congenital or cardiovascular diseases, and patients scheduled for cardiac, intracranial, or major vascular operations were excluded from the study. After induction of general anesthesia, patients were placed in a supine position with their arms positioned next to the trunk. The SCV was located with a linear probe using a method implemented previously [4]. Briefly, the longitudinalsection image of the SCV was obtained with an 18 MHz linear probe (LA435: 6–18 MHz, Esaote, Genova, Italy). Images were obtained by a single physican after acquiring a view in which the SCV seemed to have the largest diameter and the clavicle was positioned in the center of the image. Images were obtained three times on both sides at the end-expiratory phase of ventilation and were measured by two different study participants using image analysis software (MyLab TM Desk, Esaote, Genova, Italy). The internal diameter of the SCV was measured directly laterally and medially to the clavicle shadow, and defined as the average of these two values. The average of the values measured by the two different study participants was used as the SCV diameter in each pediatric patient. For statistical analysis, variables of regression modeling were selected using the stepwise selection method. R 3.1.0 (R Core Team, Vienna, Austria) was used as the statistical software, and variables were selected using the faraway and leaps packages. The leaps package automatically selects variables, and the Bayesian information criterion was used as the selection criterion. Sex, height, weight, and age were selected as independent variables