Background: Few studies have investigated the effects of preoperative dexamethasone in acute surgical patients. This study examined the effects of 8 mg dexamethasone administered intravenously 30 min before surgery for suspected acute appendicitis. Methods: A multicentre, parallel-group, double-blind, placebo-controlled study was conducted at two university hospitals in Denmark. Adults undergoing laparoscopic surgery for suspected appendicitis were eligible for inclusion. Participants, healthcare staff and investigators were blinded until all data analysis had been done. The primary outcome was the incidence of postoperative nausea and vomiting (PONV) during the first postoperative day. Secondary outcomes were pain, fatigue, sleep, opioid consumption, use of antiemetics, quality of recovery and duration of convalescence. Analysis was done according to the intention-to-treat principle. Results: A total of 120 patients were enrolled; 57 patients in the dexamethasone group and 59 in the placebo group were eligible for primary analysis. In the dexamethasone group, 47 (95 per cent c.i. 35 to 60) per cent of patients experienced PONV compared with 63 (50 to 74) per cent) in the placebo group. The absolute risk reduction in PONV was 15 (–3 to 33) per cent in favour of the dexamethasone group (P = 0·098). Patients in the dexamethasone group had less pain at rest (difference in score on visual analogue scale (VAS) 9 (95 per cent c.i. 1 to 17) mm; P = 0·024), were less fatigued (difference in VAS score 7 (0 to 14) mm; P = 0·038), used fewer opioids (absolute risk reduction 17 (2 to 33) per cent; P = 0·033) and had better quality of recovery (difference in QoR-15 score 13 (4 to 22); P = 0·006) during the first postoperative day. There was no difference in postoperative complications (P = 0·595). Conclusion: Preoperative dexamethasone did not reduce PONV by the target level of 50 per cent. Registration number: NCT02415335 (http://www.clinicaltrials.gov).