BACKGROUND: Peri-acetabular osteotomy is the joint-preserving treatment of choice in young adults with hip dysplasia but is associated with intense pain and high opioid consumption postoperatively. OBJECTIVES: To investigate whether 48 mg of pre-operative dexamethasone was superior to a standard dose of 8 mg on reducing pain in the immediate postoperative phase. DESIGN: A randomised, double-blind trial. SETTING: Single-centre, primary facility. May 2017 to August 2019. PATIENTS: At least 18 years undergoing peri-acetabular osteotomy. INTERVENTIONS: Patients were randomised 1 : 1 to 48 or 8 mg dexamethasone intravenous (i.v.) as a single pre-operative injection. All patients received a standardised peri-operative protocol, including pre-operative acetaminophen and gabapentin, total i.v. anaesthesia and local anaesthetic catheter based wound administration. MAIN OUTCOME MEASURE: Number of patients with moderate/severe pain [>3 on a numeric rating scale (NRS)] in the immediate postoperative phase. RESULTS: Sixty-four patients (32 in each group) were included, and their data analysed. At some point from tracheal extubation until transfer to the ward, the NRS was more than 3 in 75% (24/32) of the 48 mg group and in 66% (21/32) in the 8 mg group, odds ratio 1.571 (95% CI, 0.552 to 4.64), P = 0.585. Patients in the 48 mg group received less opioid [cumulative rescue analgesics, oral morphine equivalents (OMEQ)] during postoperative days 0-4: median [IQR] OMEQ was 36 [15 to 85] mg vs. 79 [36 to 154] mg in the 48 and 8 mg group, respectively, P = 0.034. There were no statistically significant differences regarding complications, rate of infections or readmissions. CONCLUSION: Forty-eight milligram of dexamethasone did not reduce pain in the immediate postoperative phase compared with an 8 mg dose. We observed insignificantly lower pain scores and significantly lower cumulated opioid requirements in the 48 mg group during the first four postoperative days. TRIAL REGISTRATION: Clinicaltrials.gov, NCT03161938, EudraCT (2017-000544-1).