Osteoarthritis of the knee is a common cause of disability in the aging population. Traditional nonoperative treatment options include exercise, physical therapy, weight reduction, nonsteroidal antiinflammatory drugs, analgesics and corticosteroid injection. In recent years, intra-articular injection of hyaluronate-derived products has gained popularity as a palliative modality for the treatment of osteoarthritis of the knee. The common adverse reaction are mild pain or swelling at the site of injection that may occur up to 20% of patients. Severe local inflammation, warmth and joint effusion are rare and no systemic complication have been reported, to our knowledge. We presented one case in which synovial inflammation was observed following intra-articular injection of the knee.