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We tested the hypothesis that ultrasound (US) guidance may reduce the minimum effective anesthetic volume (MEAV50) of 1.5% mepivacaine required to block the sciatic nerve with a subgluteal approach compared with neurostimulation (NS).After premedication and single-injection femoral nerve block, 60 patients undergoing knee arthroscopy were randomly allocated to receive a sciatic nerve block with either NS (n = 30) or US (n = 30). In the US group, the sciatic nerve was localized between the ischial tuberosity and the greater trochanter. In the NS group, the appropriate muscular response (foot plantar flexion or inversion) was elicited (1.5 mA, 2 Hz, 0.1 ms) and maintained to ≤0.5 mA. The volume of the injected local anesthetic was varied for consecutive patients based on an up-and-down method, according to the response of the previous patient. The initial volume was 12 mL. An independent observer evaluated the occurrence of complete loss of pinprick sensation and motor block: positive or negative responses within 20 min after the injection determined a 2-mL decrease or increase for the next patient, respectively.The mean MEAV50 for sciatic nerve block was 12 mL (95% confidence interval [CI], 10–23 mL) in Group US and 19 mL (95% CI, 15–23 mL) in Group NS (P < 0.001). The effective dose in 95% of cases was 14 mL (95% CI, 12–17 mL) in Group US and 29 mL (95% CI, 25–40 mL) in Group NS (P = 0.008).US provided a 37% reduction in the MEAV50 of 1.5% mepivacaine required to block the sciatic nerve compared with NS.