The traditional method to evaluate adequacy of the block for surgery is based on loss of sensory response to stimuli, which requires patient cooperation. Several methods have been described for objective assessment of the nerve block. The aim of the study was to investigate whether perfusion index (PI), a measure of peripheral perfusion from a pulse oximetry finger sensor, is a reliable and objective method for assessing the adequacy of infraclavicular blockade and to describe the time course of PI changes once peripheral nerve block has been achieved during surgery. The study was performed on patients scheduled for elective hand, wrist and forearm surgery under infraclavicular brachial plexus block. The pulse oximetry sensor was affixed to a finger ipsilateral to the side of the infraclavicular block for continuous measurement of PI. The average PI and the average percent change in PI from baseline, at 10, 20 and 30 min from the administration of the block were calculated. Baseline values of PI ranged from 0.6 to 4.7 % in 44 patients for whom infraclavicular block was effective and 1.8 to 2.4 % in 2 patients for whom infraclavicular block failed. Differences were not significant (p = 0.60). In the effective infraclavicular block group, PI rose continuously during the 30-min observation period. At 10 min, PI increased by (mean +/- A standard deviation) 120 +/- A 119 % from baseline. At 20 and 30 min, perfusion index increased by 133 +/- A 125 % and 155 +/- A 144 % from baseline. All changes from baseline were significant (p < 0.01). The perfusion index is a predictor of infraclavicular block success. The largest changes in PI occur 30 min after the block administration but significant changes in PI were detected 10 min after administration. Perfusion index monitoring may provide a highly valuable tool to quickly evaluate the success of regional anesthesia of the upper extremity in clinical practice.