Background Surgeons perform THA to address a variety of conditions in younger patients, including osteoarthritis (OA), osteonecrosis, inflammatory arthritis, and congenital deformities. Younger patients aged 50 years or younger have been characterized as active in the literature, but a direct relationship between age and activity level has not been well substantiated. Younger patients with OA may engage in higher activity levels; however, associated medical conditions in patients with other surgical indications may not support a generalization that age is a surrogate for activity level. We recently evaluated these issues in younger patients undergoing total knee arthroplasty (TKA) and noted that the majority would not be considered active. Given this observation, we considered whether younger patients undergoing THA are characterized by high activity levels, which is relevant to understanding the long-term risk of wear-related failures.Questions/purposes (1) Do demographic features of younger patients undergoing THA support high activity expectations? (2) Do preoperative or postoperative functional activity measures support projections that younger patients are active after THA? Methods We retrospectively compared demographic characteristics and functional activity profiles (as determined by preoperative and postoperative UCLA activity scores, Harris hip scores [HHS], and SF-12 and WOMAC physical function subscores) of 704 patients who had undergone THA and were aged younger than 50 years (822 hips) with those of 484 patients (516 hips) aged between 65 and 75 years, who had undergone THA, with a minimum followup at 1 year after surgery (range, 12-160 months). Results Compared with patients aged 65 to 75 years, younger patients undergoing THA were more often men (51%, 95% confidence interval [CI], 48.8%-53.2% versus 40%, 95% CI, 37.1%-42.9% women; p \ 0.01) or had undergone surgery for osteonecrosis (29% versus 4%; 95% CI, 2.8%-5.