The authors made an open multicentre clinical study with the administration of hyaluronic acid (Hyalgan(R) - Fidia) in patients with gonarthritis. The study comprised 31 patients with gonarthritis grade II-III according to Kellgren, 30 of whom completed the study. Hyalgan was administered to the patients - vials á 2 ml in five injections in weekly intervals by the intraarticular route. The patients were followed up for another three months after completed treatment. A significant decline of pain on the visual analogue scale (VAS) was recorded already two weeks after onset of treatment (p = 0.001) and this decline persisted for another 13 weeks after termination of treatment. The algofunctional indices (Lequesne, Jezek) also declined after the first injection, whereby a statistically significant reduction was recorded still after 12 weeks, as compared with values before the onset of treatment (p = 0.001). Similar results were obtained also in objective evaluations (effusion, temperature above joint, tenderness). Already after the second injection a significantly shorter time was required for a 20 m walk. The mean daily paracetamol consumption declined from a mean value of 1496 +/- 777 mg before administration to 670 6 661 mg at the end of the investigation (p = 0.00006). Undesirable effects (increased intensity of pain after puncture) was recorded in one patient (3.3%). Evidence was provided that Hyalgan(R) belongs as to its profile of effectiveness among so-called SYSADOAs (symptomatic slow acting drugs for OA). Treatment is quite safe. Key words: knee osteoarthritis, hyaluronic acid, i. a. treatment.