Objectives: To study function outcomes and their predictors after primary total hip arthroplasty (THA). Design: Prospective cohort study. Setting: Mayo Clinic. Participants: All patients who underwent primary THA at the Mayo Clinic between 1993 and 2005 and were alive at the time of follow-up. Measurements: Whether sex, age, body mass index (BMI), comorbidity, anxiety, and depression predict moderate to severe activity limitation (limitation in â¥3 activities) and complete dependence on waling aids 2 and 5 years after primary THA was examined. Multivariable logistic regression adjusted for operative diagnosis, American Society of Anesthesiologists score, implant type, and distance from medical center. Results: At 2 years, 30.3% of participants reported moderate to severe activity limitation; at 5 years, 35% of participants reported moderate to severe activity limitation. Significant predictors of moderate to severe activity limitations at 2-year follow-up were female sex (odds ratio (OR)=1.2, 95% confidence interval (CI)=1.1-1.4), aged 71 to 80 (OR=2.0, 95% CI=1.6-2.5), aged 80 and older (OR=4.5, 95% CI=3.4-6.0), depression (OR=2.1, 95% CI=1.6-2.7), and BMI greater than 30.0. At 5-year follow-up, significant predictors were aged 71 to 80 (OR=1.7, 95% CI=1.3-2.2), older than 80 (OR=4.3, 95% CI=2.8-6.6), depression (OR=2.3, 95% CI=1.6-3.4), and BMI greater than 30.0.Significant predictors of complete dependence on walking aids at 2 years were female sex (OR=2.0, 95% CI=1.4-2.7), aged 71 to 80 (OR=2.4, 95% CI=1.4-4.2), older than 80 (OR=11.4, 95% CI=6.0-21.9), higher Deyo-Charlson score (OR=1.5, 95% CI=(1.1-1.2) for 5-point increase, depression (OR=2.0, 95% CI=1.2-3.4), and BMI greater than 35.0. Each of these factors also significantly predicted complete dependence on walking at 5-year follow-up, with similar odds ratios, except that BMI of 30.0 to 34.9 was not significantly associated. Conclusion: Higher BMI, depression, older age, and female sex predict activity limitation and complete dependence on walking aids 2 and 5 years after primary THA. © 2010, The American Geriatrics Society.