Osteoarthritis is characterized by loss of articular cartilage, which has a highly limited capacity to heal itself. Knee osteoarthritis is a common but often difficult problem to manage in primary care. Recently, intraarticular hyaluronic acid injectionsW have become frequently used in patients with knee osteoarthritis. Intraarticular hyaluronic acid injections have become frequently used in patients of the synovial fluid and protect the joint cartilage. Adverse local reactions to intraarticular hyaluronic acid injections have been reported in some clinical series. However, its mechanism and long term sequelae are unclear. One potentially serious adverse event is joint infection. We experienced two cases of septic arthritis after intra-articular hyaluronic acid injections in osteoarthritis patients. When performing intraarticular hyaluronic acid injections, meticulous hygiene measures should be emphasized in osteoarthritis patients, especially for the elderly and high risk group of septic arthritis, because it is an invasive and repeated procedure.