BACKGROUND: The aim of the present study was to investigate the effect of preemptive gabapentin on postoperative pain and opioid consumption in patients undergoing mastectomy. METHODS: In a randomized, double-blind study, 41 patients received a single dose of oral gabapentin 900 mg with routine premedicantion (GABA group, n = 21) or only routine premedication without gabapentin (control group, n = 20) 1 h before surgery. Patients received intravenous patient-controlled analgesia using fentanyl for postoperative analgesia. VAS scores for pain at rest and during movement, sedation scores, side effects, and fentanyl consumptions were assessed at 6 and 24 h after surgery. RESULTS: No significant differences were found between the two groups in terms of pain scores, side effects, or fentanyl consumptions for 24 h postoperatively. However, patients in the GABA group were more sedated at 6 h postoperatively than patients in the control group (P < 0.05). CONCLUSIONS: The preemptive administration of oral gabapentin at 900 mg did not reduce postoperative pain or fentanyl consumption after mastectomy during the 24 h period following surgery.