Arthroscopy of the knee, including synovial biopsy under direct vision, was performed in 22 patients (21 cases of arthritis, 1 of osteoarthrosis), treated 15 days to 12 months previously by osmic or isotopic synoviorthesis. The aspects seen after the 16 failures were compared with those seen after good results (4 cases) and after recurrences (2 cases). In arthritis, failure was due either to persistent synovitis, to synovial necrosis or to a combination of the two. Arthroscopy is useful since the discovery of inflammatory lessions justifies repeat synoviorthesis, whilst necrosis indicates the need for articular lavage.