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Controversy exists regarding the need for nerve stimulation when performing an ultrasound (US)-guided peripheral nerve block. We tested the hypothesis that the quality of a femoral nerve block (FNB) performed with US is equivalent to an FNB performed with US and nerve stimulation.One hundred seven patients undergoing unilateral total knee arthroplasty were randomized to receive either a US-guided FNB (group US) or a US-guided FNB with nerve stimulation (group USNS). Thirty milliliters of bupivacaine 0.5% was injected in both groups. At 10, 20, 30, and 40 mins after block placement, blinded motor and sensory examinations were conducted. Secondary outcomes included time to perform the block, the number of needle redirections, and 24-hrs intravenously administered morphine equivalent consumption.There were no significant differences in the proportion of patients with either a partial or complete block. At 40 mins, 95.7% of the USNS subjects had a partial or complete sensory block of the femoral nerve (complete in 71.7% and partial in 24%) compared with 88.1% of US subjects (complete in 69% and partial in 19.1%; odds ratio, 2.97; P = 0.19). There were more needle redirections in group USNS (4.1 vs 1.1, P < 0.001), with a higher percentage of patients requiring 2 or more needle attempts (44.2% vs 18.9%, P < 0.01). The time to perform the block in group USNS was longer (188 vs 148 secs, P = 0.01).The addition of nerve stimulation to a US-guided FNB did not change preoperative block efficacy.