© 2015, American College of Rheumatology. Objective To examine whether an underlying diagnosis of rheumatoid arthritis (RA) or osteoarthritis (OA) impacts the 90-day readmission rates after total hip arthroplasty (THA) or total knee arthroplasty (TKA). Methods We analyzed prospectively collected data from an integrated health care system, Total Joint Replacement Registry, of adults with RA or OA undergoing unilateral primary THA or TKA during 2009-2011. Adjusted logistic regression models for 90-day readmission were fit. Odds ratios with 95% confidence intervals (95% CIs) were calculated. Study year was an effect modifier for the outcome; therefore separate analyses were conducted for each of the 3 study years. Results Of the 34,311 patients, 496 had RA and 33,815 had OA. Comparisons of RA and OA patients, respectively, were 73% and 61% women, 45% and 70% white, and patients had a mean age of 61 versus 67 years (P < 0.001). Crude 90-day readmission rates for RA and OA were 8.5% and 6.7%, respectively. The adjusted odds of 90-day readmission increased from year to year for RA compared to OA patients, from 0.89 (95% CI 0.46-1.71) in 2009 to 1.34 (95% CI 0.69-2.61) in 2010, and to 1.74 (95% CI 1.16-2.60) in 2011. The 2 most common readmission reasons were joint prosthesis infection (10.2%) and septicemia (10.2%) in RA and joint prosthesis infection (5.7%) and other postoperative infection (5.1%) in OA. Conclusion RA is a risk factor for 90-day readmission after primary THA or TKA. An increasing risk of readmissions noted in RA in 2011 is concerning and indicates that further studies should examine the reasons for this increasing trend.