The characterization of synovial tissue in rheumatoid arthritis (proliferative or exsudative synovitis), which is important in deciding between local therapies of arthritis, was examined by arthrosonography, computerized axial tomography and arthroscopy. The arthroscopic examination yields the highest significance in characterizing rheumatoid synovial tissue. Compared to arthroscopy, arthrosonographic diagnosis was correct in 70% of our cases. The arthrosonographic results were much influenced by the different concentrations of fibrin in the synovial effusions.