Background and purpose - Larger prospective studies investigating periacetabular osteotomy (PAO) with patient-reported outcome measures developed for young patients are lacking. We investigated changes in patient-reported outcome (PRO), changes in muscle-tendon pain, and any associations between them from before to 1 year after PAO. Patients and methods - Outcome after PAO was investigated in 82 patients. PRO was investigated with the Copenhagen Hip and Groin Outcome Score (HAGOS). Muscle-tendon pain in the hip and groin region was identified with standardized clinical tests, and any associations between them were analyzed with multivariable linear regressions. Results - HAGOS subscales improved statistically significantly from before to 1 year after PAO with effect sizes ranging from medium to very large (0.66-1.37). Muscle-tendon pain in the hip and groin region showed a large decrease in prevalence from 74% (95% CI 64-83) before PAO to 35% (95% CI 25-47) 1 year after PAO. Statistically significant associations were observed between changes in HAGOS and change in the sum of muscle-tendon pain, ranging from -4.7 (95% CI -8.4 to -1.0) to -8.2 (95% CI -13 to -3.3) HAGOS points per extra painful entity across all subscales from before to 1 year after PAO. Interpretation - Patients with hip dysplasia experience medium to very large improvements in PRO 1 year after PAO, associated with decreased muscle-tendon pain. The understanding of hip dysplasia as solely a joint disease should be reconsidered since muscle-tendon pain seems to play an important role in relation to the outcome after PAO.