Intraarticular sodium hyaluronate injection to treat osteoarthritis is associated with minor side effects. Infections seem uncommon. We report two cases of septic knee arthritis. One patient was an 80-year-old woman who was admitted for Staphylococcus aureus knee arthritis after several intraarticular injections of sodium hyaluronate and corticosteroids. In the other patient, a 78-year-old woman, Neisseria mucosa knee arthritis occurred after a single sodium hyaluronate injection. Faultless aseptic technique is essential when administering hyaluronate viscosupplementation. Patients should be informed of the risk of septic arthritis.