Because implants can provoke varied immune system responses, we assessed whether hip and knee implant recipients had an increased risk of autoimmune/connective tissue diseases (AI/CTDs). Using national registry data from Sweden, we compared hospitalization rates for AI/CTD in 101,771 hip and 23,891 knee implant recipients to rates in the general population. Hip patients were followed up to 22 years and knee patients up to 14 years postimplantation. Our findings indicate that it is unlikely that hip or knee implantation results in any increased risk for most AI/CTDs. After long-term followup, the associations we observed with polyarteritis nodosa and fibrositis could be the basis for future investigations.