In 66 patients with rheumatoid knee joint synovitis and hydrops and with only slight radiological destruction, local treatment of the knee was randomly performed either with osmic acid, radioactive yttrium or surgical synovectomy. After a one-year follow-up the clinical and radiological results were slightly better in patients who underwent surgical synovectomy than in other treatment groups. However, radiological osteoarthrosis had progressed more in synovectomized patients than in others. In 2 patients with radioactive yttrium a relapse requiring surgical synovectomy was seen. Because osmic acid is almost as effective as surgical synovectomy, is very cheap and easy to perform, it can be recommended as the first choice for local therapy in patients with corticosteroid-restant knee joint synovitis, in the early stage of the disease.