BACKGROUND Ultrasound guidance during peripheral nerve blocks has allowed for reduction in dose and volume of local anesthetic required to accomplish successful blockade using multiple injections through a needle. The authors undertook this study to determine the minimal effective volume required to accomplish successful interscalene brachial plexus block (ISB) through the catheter. METHODS After obtaining institutional ethics committee approval and written informed consent, patients aged 18-75 yr and scheduled for arthroscopic shoulder surgery under ISB were enrolled. All patients using a step-up/step-down method and the starting dose of 15 ml of 0.75% ropivacaine received an ultrasound-guided ISB catheter. The injection volume was increased or decreased by 1 ml in case of block failure or block success, respectively. RESULTS The authors found that the proportion of patients with successful blockade increased sharply from approximately 57% at 6 ml to 100% by 7 ml, indicating that a small increase in volume of ropivacaine 0.75% markedly affects the success rate. A total of 12 ISB with injection volume less than 7 ml resulted in successful anesthesia within 30 min, yielding an ED95 of 7 ml (95% CI 6.8-7.2) ropivacaine 0.75%. For the group as a whole, the median (min-max) sensory block onset time was 5 (5-20) min, the median (min-max) motor blocks for the biceps and the deltoid muscles were 7.5 (5-15) min and 10 (5-15) min, respectively. The median (min-max) block duration was 8.9 (3-15) h. CONCLUSIONS An injection of a minimum of 7 ml of ropivacaine 0.75% through the catheter is required for success rate and timely onset of surgical anesthesia with ISB.