Intra-articular hyaluronic acid injection has a positive effect on pain and functional status of knee osteoarthritis. In this study we examined the long-term effect of sodium hyaluronate (Orthovisc) injection therapy in 81 patients who were followed up for one year. Orthovisc was injected once a week for 3 weeks. The patients with grade 4 osteoarthritis according to Kellgren-Lawrence were excluded. Visual Analog Scale and Lequesne algofunctional index were evaluated at 1, 3, 6 and 12 months. 81 patients were assessed at the 1(st) month, 80 at the 3(rd) month, 55 at the 6(th) month and 38 at the 12(th) month. Of all the patients 38 (69%) responded well to the therapy at six months and 28 patients (34.5%) at one year. Visual Analog Scale and Lequesne score decreased significantly after the injection (p<0.01). The most relevant decrease was seen at the 3(rd) month. Age, gender, duration of osteoarthritis and body mass index did not affect the results. Patients with medial compartment osteoarthritis and with grade 1 and grade 2 osteoarthritis responded significantly better to the therapy.