With over 100,000 procedures done per year, hip and knee replacements are two of the most common surgical procedures performed in Canada. Post-operatively, Patient Reported Outcome Measures (PROMs) will start to plateau with time as patients reach the limits of how a prosthetic joint can function for them. There has been literature indicating that PROMs will start to plateau between 6 and 12 months. Having an understanding of the precise timeframe, as well as the factors that can impact this trajectory, can be beneficial for setting patient expectations, and directing patient follow-up. Thus, our purpose was to analyze the trajectory of PROMs following total hip and knee arthroplasty (THA and TKA), as well as assess the impact of any potential confounders on this trajectory. This study was a retrospective analysis of data from a prospective database of a consecutive series of primary THA and TKA patients. The database is collected at a single academic arthroplasty centre, located in Ontario, Canada. Patients were eligible if they had undergone an elective, primary THA/TKA with Oxford Scores recorded pre-operatively, and at least at two of the following three time points: 6 weeks, 6 months, 1 year, and 2 years. Clinically meaningful change from one timepoint to the next was defined as 5 points, based on previously established minimal clinically important difference (MCID) values for both hip and knee scores. General linear models with repeated measures were used to assess for change between timepoints. American Society of Anesthesiologist (ASA) class and date of operation were included as potential between subject factors in the respective hip and knee models. Mean data reported along with standard deviation in parentheses. Overall, 8,276 joints were eligible for inclusion in the study. There were more female patients, with 60.4% being female (N = 5,000). Mean age was 67.9 years (9.6), while mean body mass index (BMI) was 31.6 kg/m2 (6.8). Nearly two-thirds (66.1%) of operative joints were knees (5,467/8,276). Mean pre-operative scores were 18.0 (7.8) for THA, and 20.1 (7.5) for TKA. For both THA and TKA, there were statistically significant interval improvements in Oxford scores from 6 weeks [THA: 33.8 (7.9)/TKA: 28.7 (7.8)] to 6 months [THA: 40.2 (7.3)/TKA: 35.9 (8.3)], and from 6 months to 1 year [THA: 41.0 (7.3)/TKA: 37.3 (8.4)], but not from 1 to 2 years [THA: 40.0 (8.5)/TKA: 36.4 (9.6)]. The change from 6 months to 1 year for both hip and knee patients was below the threshold for a clinically meaningful change based on an MCID value of 5.0. None of the between-subject factors identified as potential sources and bias and included in the model (BMI, ASA, or date of surgery) demonstrated a significant interaction effect. Patients undergoing both THA and TKA can expect clinically meaningful improvements up to 6 months, at which point there is a plateau in PROM scores. These findings are important for both for setting patient expectations in pre-operative discussions, and allowing surgeons to have a realistic understanding of their patients' expected post-operative course.