Objective: To investigate the risk of medical complications following total hip and knee arthroplasty (THA/TKA) among rheumatoid arthritis (RA) compared with osteoarthritis (OA) patients; and, to assess the risk of complications among biologics-treated RA patients. Methods: In a nationwide register-based study, patients with RA and OA with THA/TKA surgery between 2000 and 2015 were identified and followed up to 90 days after surgery for venous thromboembolism (VTE), myocardial infarction and stroke, and non-surgical infections, respectively. Information on treatment with biologics was obtained in the DANBIO rheumatology register to compare risks of complications with non-biologics treated. Results: A total of 2899 and 112,571 patients with RA and OA had THA/TKA. RA was associated with a hazard ratio (HR) of 1.29 (1.03 to 1.61) for infection following THA/TKA, but a HR of 0.60 (0.26 to 0.98) for VTE following TKA. Biologics treated patients had a HR of 1.35 (0.65 to 2.80) for infection and 4.82 (1.67 to 13.90) for VTE compared with non-biologics treated RA patients. RA patients had no increased risk of post-surgical myocardial infarction and stroke (HR 1.16, 0.76 to 1.78) compared with OA, but a higher incidence proportion was observed in biologics treated compared with non-biologics treated (1.0% vs 0.6%); however, the number of events were too small to estimate a HR. Conclusion: In this study, RA was a risk factor for infection after THA/TKA, and RA patients treated with biologics had a slightly increased risk compared with non-biologics treated RA patients. Compared with OA, RA patients had a lower risk of VTE following THA/TKA, but our finding of increased incidences of VTE in biologics-treated patients warrants further studies.