Background: Patient-reported outcome measures (PROMs) after total hip arthroplasty (THA), can be influenced by patient characteristics (case-mix factors). We used the Dutch Arthroplasty Register (LROI) to determine the effect of case-mix on improvement of PROMs after primary THA. Methods: We included all primary THAs (n = 22,357) performed in the Netherlands between 2014 and 2018. The Hip disability and Osteoarthritis Outcome Score Physical function short form (HOOS-PS), Oxford Hip Score (OHS), EQ-5D index score and thermometer, and Numeric Rating Scales (NRS) measuring pain during activities and at rest, were recorded. The difference between preoperative and 3- and 12-month postoperative scores was calculated (delta-PROM) and used as primary outcome variable. Multivariable linear regression was used to examine the association between patient characteristics (age, sex, ASA score, body mass index (BMI), Charnley class, smoking, and previous operations to the affected hip) and PROMs. Cohens' d was used to measure effect size. Results: Postoperative improvement (delta-PROM) on HOOS-PS, OHS, EQ-5D, and pain relief were significantly higher in patients 30 kg/m(2), and patients without a previous operation to the hip. Cohen's d indicated clinically small differences (0.2). Conclusions: Patients benefiting most in terms of postoperative improvement of self-reported physical functioning, pain relief and quality of life after primary THA were young, female, with a high ASA or BMI score, and without previous operations to the hip.