As many as 200 patients with acute rheumatic fever were examined. Impairment of the joints in the form of classic migrating polyarthritic was detected in 96% of the cases. The knee (in 80%), talocrural (in 72%), slightly rare radiocarpal (in 21%), and elbow joints were mostly damaged. In addition to the above-indicated damage to the joints, one could see slightly atypical injuries in the form of monoarthritis (in 4%), arthritis of small articulations of the feet and hands (in 6%), and of the sternoclavicular articulations (in 3.5%). According to the cytological, biochemical and immunological findings, synovial fluid obtained from 11 patients suffering from acute rheumatic fever did not differ, significantly from synovial fluid of patients with reactive arthritis.