Various parameters have been considered as possible predictors of health-related quality-of-life outcomes after THA in patients with hip osteoarthritis. We hypothesized the preintervention health status is the main and more homogeneous predictor of changes of the different aspects of health-related quality-of-life outcomes, mental health status has an important influence on results, whereas other sociodemographic or clinical factors had only a punctual influence. All patients who fulfilled the selection criteria completed the Medical Outcomes Study SF-36 and the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) before and 6 months after the intervention. Seven hundred eighty-eight patients completed the questionnaire before the intervention and 590 completed it (74.9%) at 6 months. The preintervention score in each SF-36 and WOMAC domain and the SF-36 mental health domain predicted changes after the intervention. Female gender, having comorbidities, contralateral hip osteoarthritis, or back pain predicted less improvement on some SF-36 domains. Older age, the presence of contralateral hip osteoarthritis, or back pain predicted less improvement on some of the WOMAC domains. Preintervention health status, measured by the WOMAC or SF-36, and mental health status uniformly predicted health-related quality-of-life changes, whereas some clinical parameters predicted some domains. SF-36 and WOMAC seem to be appropriate tools for predicting THA outcomes.