Surgically revised deep infected primary knee arthroplasties reported to the Swedish knee arthroplasty register during the years 1986-2000 were studied with respect to microbiology, antimicrobial susceptibility pattern and changes over time. In early, delayed and late infections, coagulase-negative staphylococci (CoNS) were most prevalent (105/299, 35.1%), and twice as common as Staphylococcus aureus (55/299, 18.4%). In haematogenous infections, S. aureus was the dominating pathogen (67/99, 67.7%), followed by streptococci and Gram-negative bacteria. Methicillin resistance was found in 1/84 tested isolates of S. aureus and 62/100 tested isolates of CoNS. During the study period, methicillin resistance among CoNS increased (p=0.002). Gentamicin resistance was found in 1/28 tested isolates of S. aureus and 19/29 tested CoNS isolates. A relative decrease in infections caused by S. aureus was observed, while enterococci increased. In empiric treatment of infected knee arthroplasty the type of infection should direct the choice of antibiotics. Awareness of the fact that most early infections are caused by CoNS can increase the chances of successful treatment with retained implant. Due to the high rate of gentamicin resistance among CoNS in infected knee arthroplasty, other antibiotics should be used in bone cement at revision.