This study tested the hypothesis that increasing the concentration of bupivacaine from 0.375 to 0.75% would increase the duration of postoperative analgesia by 3 h. Seventy patients scheduled for hospital admission after foot or ankle surgery gave consent to enter this prospective randomized trial. Patients were randomly assigned to receive a popliteal fossa block (posterior approach) using 30 cc of either 0.375% or 0.75% bupivacaine, with epinephrine. Patients also received a neuraxial anesthetic and postoperative intravenous patient-controlled analgesia. Patient characteristics, duration of analgesia, pain scores, use of analgesic medications, and side effects of analgesic therapy were determined. Duration of analgesia was similar with both concentrations of bupivacaine (0.375% 14 ± 8 h, 0.75% 13 ± 6 h; mean ± SD). Pain scores were the same for both groups on the first postoperative day (3 of 10 at rest, 5 with therapy). Analgesic use and side effects attributable to pain management did not differ between groups. In conclusion, postoperative analgesia was not affected by the concentration of bupivacaine used for the nerve block. There was no benefit to increasing the concentration of bupivacaine above 0.375% for single-injection popliteal fossa nerve blockade when performed for postoperative analgesia.