Glucocorticoids have attracted increasing attention as adjuvants in the treatment of acute postoperative pain. Furthermore, anecdotal reports may support glucocorticoids for preventing sustained postoperative pain. We explored preoperative dexamethasone combined with paracetamol and ibuprofen on acute and sustained pain after lumbar disk surgery. In this blinded study, 160 patients undergoing lumbar disk surgery were randomly assigned to 16 mg IV dexamethasone or placebo. All patients received perioperative paracetamol and ibuprofen, and postoperative IV patient-controlled analgesia with morphine. Primary outcome was pain during mobilization (visual analog scale) 2 to 24 hours postoperatively. Secondary outcomes were acute pain at rest, morphine consumption, nausea, vomiting, ondansetron consumption, sedation, and quality of sleep. Patients were followed up by written questionnaire 3 months postoperatively. Acute pain during mobilization (weighted average area under the curve, 2-24 hours) was significantly reduced in the dexamethasone group: 33 (22) mm vs placebo 43 (18) mm, (95% confidence interval [CI] 3-16) P = 0.005. Vomiting 0 to 24 hours postoperatively was reduced in the dexamethasone group (17 episodes) vs placebo (51 episodes) P = 0.036. No other differences were observed. However, 6.5% (95% CI 2-15) in the dexamethasone group vs placebo 0% had an antibiotically treated wound infection (P = 0.13). Sixteen percent (95% CI 7-26) vs 8% (95% CI 0-17) reported new weakness/paralysis of the legs in the dexamethasone and placebo groups, respectively, 3 months postoperatively (P = 0.20). In conclusion, preoperative dexamethasone significantly reduced pain during mobilization and vomiting, after lumbar disk surgery. No significant effects were observed 3 months postoperatively.