Infection after knee replacement is usually blamed on the patient. We asked whether the operation itself matters. In 2920 knee replacements from one New Zealand hospital, each added feature of a complex operation, such as a previous implant, a bone deformity or a larger, stemmed implant, raised the risk of later infection by about 60 %. Leg alignment, which affects wear and loosening, did not change infection risk. Surgeons should weigh how complex the operation will be when judging this risk.