We evaluated the optimal preemptive dose of gabapentin for postoperative pain relief after single-level lumbar diskectomy and its effect on fentanyl consumption during the initial 24 hours in a randomized, double-blinded, placebo-controlled study in 100 patients with American Society of Anesthesiologists physical status I and II. Patients were divided into five groups to receive placebo or gabapentin 300, 600, 900, or 1200 mg 2 hours before surgery. After surgery, patients were transferred to the postanesthesia care unit (PACU). A blinded anesthesiologist recorded the pain scores at time points of 6, 12, 18, and 24 hours in the PACU on a Visual Analog Scale (VAS; 0-10 cm) at rest. Patients received patient-controlled analgesia (fentanyl 1.0 μg/kg on each demand with lockout interval of 10 minutes); total fentanyl consumption during initial 24 hours was recorded. Data were entered into the statistical software package SPSS 9.0 for analysis (one-way analysis of variance and Student-Newman-Keuls test). Patients who received gabapentin 300 mg had significantly lower VAS score at all time points. They consumed less fentanyl (patients who received placebo processed 1217.5 ± 182.0 versus 987.5 ± 129.6 μg; P < 0.05). Patients who received gabapentin 600, 900, and 1200 mg had lower VAS scores at all time points than patients who received gabapentin 300 mg (P < 0.05). Increasing the dose of gabapentin from 600 to 1200 mg did not decrease the VAS score, nor did the increasing dose of gabapentin significantly decrease fentanyl consumption (702.5, 635, and 626.5 μg). Thus, gabapentin 600 mg is the optimal dose for postoperative pain relief following lumbar diskectomy.