The recommended needle trajectory for the vertical infraclavicular brachial plexus block is anteroposterior, caudad to the middle of the clavicle. We studied the risk of pneumothorax and subclavian vessel puncture and the precision of this method by using magnetic resonance imaging in 20 adult volunteers. The trajectory aimed at the lung in six subjects, five of whom were women. However, pleural contact could be avoided in all subjects by halting needle advancement after contact with the subclavian vessels, plexus, or first rib. The subclavian vein was reached by the trajectory in three and the subclavian artery in five subjects. The trajectory had a median distance to the plexus (closest aspect) of 1 mm (range, 0-9 mm) and contacted the nerves in 9 subjects. In conclusion, there is a small probability that the needle may reach the pleura when a vertical infraclavicular brachial plexus block is performed, particularly in women, and a high probability that it will contact the subclavian vein or artery. Although the trajectory is close to the plexus, any medial deviation carries the risk of pleural or subclavian vessel contact at other depths. Clinical accuracy in defining the insertion point is critical. © 2005 by the International Anesthesia Research Society.