OBJECTIVE: Patient age has been reported to be an important determinant of treatment success in developmental dysplasia of the hip (DDH). This study aimed to determine the efficacy of periacetabular osteotomy (PAO) in DDH patients aged >= 35 years and to compare patient-reported and radiological outcomes with a control cohort of younger patients. PATIENTS AND METHODS: This retrospec-tive analysis included 43 patients (23 aged < 35 years; 20 aged >= 35 years) who underwent uni-lateral Bernese PAO for symptomatic DDH, be-tween May 2001 and August 2015. Patients with a history of ipsilateral pelvic osteotomy or oth-er pelvic pathologies were excluded. To evalu-ate the morphology of the dysplastic hip joints, 11 conventional radiographic measures were taken. Preoperative and postoperative Harris Hip Scores, postoperative visual analogue scale scores, Western Ontario and McMaster Universi-ties Osteoarthritis Index (WOMAC) scores, and hip range of motion measurements were used for clinical evaluation. RESULTS: Of the 43 patients, 31 (72.1%) were women and 14 (27.9%) were men. The average age was 32 (range: 19-45) years. Regarding sex, surgery site, mean body mass index and mean follow-up time, there were no significant differ-ences between the groups. Both groups showed significant differences in radiographic parame-ters before and after the PAO. There were no sig-nificant differences in preoperative or postoper-ative clinical outcomes between the groups, ex-cept for the WOMAC score. CONCLUSIONS: PAO can be performed safe-ly to treat acetabular dysplasia in patients aged >= 35 years, before the onset of femoroacetabu-lar osteoarthritis, with satisfactory clinical and functional outcomes.