Perioperative factors associated with late septic arthritis after knee and hip arthroplasties were prospectively investigated. All patients received a short course of perioperative cefuroxime. After a follow-up of 1 year, septic arthritis was diagnosed in 9/362 patients (2.5 percent) after knee arthroplasty and in 17/2651 patients (0.64 percent) after hip arthroplasty. For the knee, factors associated with septic arthritis after arthroplasty were rheumatoid arthritis, wound infection, an unhealed wound, and a painful, limited knee function at discharge from the hospital. For the hip, corresponding risk factors were diabetes, failed fracture osteosynthesis, a breakdown of sterility during operation, wound infection, postoperative urinary tract infection, and an unhealed wound at discharge from the hospital or a difficult rehabilitation course. Reoperation after knee and hip arthroplasty was also clearly associated with a higher incidence of septic arthritis.