In this study, 30 percutaneous synovial knee biopsies were taken from patients with systemic lupus erythematosus (SLE) that had been diagnosed in accordance with ARA criteria. Acute articular involvement was present in 16 cases. The synovial membrane was then examined using routine histological techniques and by direct immunofluorescent methods. The most frequently observed histological picture was comprised of: a) synoviocyte hyperplasia, b) scarce inflammatory infiltrate, c) vascular proliferation, d) oedema and congestion, e) fibrinoid necrosis and intimal fibrous hyperplasia of blood vessels, g) presence of fibrin on the synovial surface and fibrin-like deposits in the chorion. It is concluded that immunofluorescence can be used in the characterization of typical cases but is not specific.