Intra-articular hyaluronic acid injections in the case of osteoarthritis of the knee may restore the viscous properties of the synovial fluid and protect the joint's cartilage. In theory the hyaluronic acid has an analgesic and anti-inflammatory effect as well as a favourable influence on chondrocyte metabolism. In six published studies which compared the effectiveness of hyaluronic acid with injection of a placebo of a non-steroidal anti-inflammatory medication, the hyaluronic acid injections had a relatively long-term favourable effect (compared with corticosteroid injections) on pain, function, serous gonitis and concomitant analgesic use. Just as for corticosteroid injections, the placebo effect was large. Intra-articular hyaluronic acid injections have few side effects. However, in general three to five injections, with weekly intervals, are needed for the effect to be achieved. Furthermore, the medication is relatively expensive. Future investigations will be necessary to provide insights into both the cost-benefit aspects and the long-term effects of the medication.