The aim of this study was to determine the minimum effective concentration of ropivacaine for ultrasound-guided supraclavicular brachial plexus block. Fifty-one patients undergoing arm surgery received double-injection ultrasound-guided supraclavicular block using ropivacaine 40 ml. The concentration of ropivacaine administered to each patient started at 0.225% and then depended on the response of the previous one, based on a biased coin design up-and-down sequential method. In case of failure, the ropivacaine concentration was increased by 0.025% w/v in the next subject. In the case of a successful block, the next patient was randomised to the same concentration or a concentration 0.025% w/v less. Success was defined as complete sensory blockade of the brachial plexus 30 min after the block together with pain-free surgery. The minimum effective ropivacaine concentration in 90% of subjects was 0.257% w/v (95% CI 0.241-0.280%).