Objective:Accidental puncture of the vertebral artery during the internal jugular vein cannulation produces lethal sequelae. To prevent this, the cannulation needle must not he inserted too deeply, However, there is no useful guide for the optimal length of insertion of the needle for accessing the internal jugular vein. The authors examined the length of the needle needed to reach the internal jugular vein with three different sizes of needle (16, 20, and 23 gauge). Design: Prospective study. Setting: An academic medical center. Participants: Patients undergoing cardiovascular surgeries. Interventions: The cannulation of the internal jugular vein was performed through the right internal jugular vein by the high approach. The needle was slowly advanced, keeping constant negative pressure on the syringe at 45 degrees to the skin surface until blood was aspirated; if blood was not aspirated during insertion, the needle was slowly withdrawn until blood was aspirated, The distance to the internal jugular vein was assessed by calculating the entire length of needle minus the length of needle from the skin surface to the hub. Measurements and Main Results: The mean distance to the internal jugular vein ranged from 15.0 to 21.5 mm, and the larger needle required the longer distance to the internal jugular vein. Conclusions: The results may be a useful guide to prevent too deep insertion of the needle during internal jugular vein catheterization, especially when teaching residents who have limited experience with internal jugular Vein catheterization. Copyright (C) 1997 by W.B. Saunders Company.