BACKGROUND: Despite advances in surgical and anesthesiology techniques, many patients continue to experience postoperative pain after lumbar disc surgeries. The aim of this study was to investigate the effect of preoperative oral melatonin on the severity of postoperative pain after lumbar laminectomy/discectomy. METHODS: In this double blinded randomized controlled clinical trial 80 patients undergoing an elective mini-open microdiscectomy surgery at Imam Khomeini educational hospital in Sari, Iran, were selected and randomly assigned into one of four groups. Patients in group A, B, C, and D received 3, 5 and 10 mg melatonin or placebo tablets one hour before surgery, respectively. Using the visual analogue scale (VAS) the severity of pain, nausea and vomiting, pruritus, and use of narcotics were assessed immediately after surgery and before leaving the post-anesthesia care unit, 6, 12 and 24 hours postoperatively. RESULTS: In all three groups receiving melatonin at all three different doses, postoperative pain was significantly less than the placebo group (P<0.01). There were no statistically significant differences in postoperative pain level between the three groups receiving melatonin (P>0.05). The amount of opioid received by the patients within 24 hours after surgery had statistically significant differences within the groups (P=0.043, F=2.58). The results of post hoc analysis in terms of postoperative pain intensity showed statistically significant differences between the two groups receiving melatonin at a dose of 5 mg and the placebo group (P=0.04). No serious side effects reported in four groups. CONCLUSION: The use of oral melatonin with a dose of 5 mg, 1 hour before the surgery as an inexpensive method can effectively reduce pain intensity as well as the amount opioid use after lumbar laminectomy and discectomy.