BACKGROUND: Reoperations following total hip arthroplasty (THA) remain a major clinical challenge, with their incidence and socioeconomic burden rising despite advances in surgical techniques and prosthesis design. This study aimed to evaluate the predominant causes of reoperations following THA over two decades at a tertiary referral hospital, comparing trends between 2004 and 2013, and 2014 and 2023. METHODS: We analyzed all reoperations performed at a tertiary referral hospital between January 2004 and December 2023. A total of 515 hips (483 patients) were included after excluding multiple reoperations on the same hip. The causes of reoperation were stratified into two time periods (2004 to 2013 and 2014 to 2023) to analyze trends. The time interval from primary THA to reoperations was also evaluated. RESULTS: The main cause of reoperation after primary THA was aseptic loosening, accounting for 52.4% of cases, followed by infection (13.2%), periprosthetic fracture (PPF) (10.7%), wear/osteolysis (8.5%), ceramic fracture (5.8%), and instability/dislocation (5.6%). The proportion of aseptic loosening decreased significantly from 62.5 to 40.4%, while the proportions of infection, PPF, ceramic fracture, and instability/dislocation increased (P <
0.001). The causes of reoperation varied according to the time interval. Instability, PPF, and infection were early causes, and wear/osteolysis and aseptic loosening were relatively later causes of reoperations. CONCLUSION: Aseptic loosening was the most common cause of reoperation following primary THA. However, the proportion of infection and PPF increased as a cause of reoperations, while the proportion of aseptic loosening decreased with time. Surgeons should consider that the main cause of reoperations differed according to the time interval from primary THA to reoperation.