A double-blind, placebo-controlled study was carried out in 34 patients suffering from osteo-arthritis of the knee. A total of 40 joints was treated at random with 3 intra-articular injections, at 1 week intervals, of either 20 mg sodium hyaluronate or placebo. Clinical examinations, including assessments of spontaneous pain intensity, pain on touch, under load and while walking, were made before each injection and repeated 7 days after the last one and again at 60 days after the start of the trial. The results showed a significant difference between treatments for all the variables assessed. In the sodium hyaluronate group, pain relief was not only rapid but also long lasting. Local tolerance was very good for both treatments.