AIMS: Since April 2009, patients undergoing NHS-funded primary elective total knee or hip arthroplasties (TKAs or THAs) should be invited to complete patient-reported outcome measures (PROMs). The aim of this study was to use National Joint Registry (NJR) data to identify patient and demographic differences between those with complete versus those with missing PROMs. METHODS: Patients undergoing publicly funded elective primary TKA or THA between April 2009 and December 2018 recorded in the NJR were eligible. NJR data were linked to the English Hospital Episode Statistics (HES), and PROMs data. Oxford Knee/Hip scores (OKS/OHS) were eligible for inclusion if recorded up to 18 weeks preoperatively (Q1) and between six and 12 months postoperatively (Q2). Proportions were used to describe the completeness of PROMs data. The following variables were assessed to determine associations with completeness of PROMs data: age, sex, American Society of Anesthesiologists (ASA) grade, BMI, socioeconomic deprivation, baseline (Q1) OKS/OHS, and hospital. RESULTS: Of 570,449 eligible TKAs and 507,962 eligible THAs, complete pre- and postoperative PROMs were available for 229,794 TKAs (40.3%) and 210,929 THAs (41.5%). There were no differences in the patient and demographic factors between those with and without complete PROMs. Patients with complete PROMs had higher preoperative PROMs (less pain and functional limitations) than those with preoperative but no postoperative PROMs (TKA: median OKS = 19 vs 15; THA: median OHS = 17 vs 14). For Q2 PROMs, the median hospital-level completeness was 71% OKS (IQR 60 to 76) and 72% OHS (IQR 62 to 77), with little variation between high and low volume hospitals. In contrast, Q1 completeness varied between hospitals (69% in high-volume hospitals vs 57% in low-volume hospitals). CONCLUSION: Although up to 60% of patients did not have complete pre- and postoperative PROMs, the patient and demographic factors, and the rates of revision and mortality, were similar to those with complete PROMs. PROMs which are collected may therefore probably be generalizable to wider groups of patients who undergo TKA and THA.