Introduction: Little is known about muscle strength in patients undergoing periacetabular osteotomy (PAO). Therefore, we investigated muscle strength in patients with hip dysplasia before and after PAO and compared this to healthy controls. Furthermore, we investigated if pre- to post-surgery changes in patient-reported pain and sporting function were associated with changes in muscle strength. Material and methods: Muscle strength was assessed twice in 100 patients, before and 1 year after PAO, and once in 50 controls. Isometric muscle strength was assessed in hip flexion, extension, abduction and adduction with a hand-held dynamometer. Hip and Groin Outcome Score (HAGOS) was used to measure pain and sporting function. Differences between patients and controls, and changes in muscle strength from before to after PAO were analysed with a mixed effect model, while associations between changes in HAGOS and changes in strength were analysed with linear regressions. Results: Despite small improvements in hip flexion and abduction (0.1 Nm/kg (CI 0.06-0.2) and 0.1 Nm/kg (CI 0.02-0.2) 1 year after PAO, the muscle strength of the patients were 13-34% lower than the muscle strength of the controls, pre- and post-surgery (p