Objective. The aim of this study was to assess inflammatory changes within the knee joint of systemic lupus erythematosus (SLE) patients by using ultrasound (US). Rheumatoid arthritis (RA) patients and healthy subjects (HS) were evaluated as controls. US,findings were correlated with disease activity parameters. Methods. Twenty-six SLE patients were enrolled in the study, 25 RA patients and 15 HS were selected as controls. US was performed by two different experienced operators, using an Agilent-HP Image point Hx machine equipped with a 10 MHz linear transducer. Power Doppler (PD) was used to determine local synovial perfusion (PFR 700-1100 Hz; gain 60-65dB; low filter). Knee joints were examined bilaterally. US findings, expressed after consensus of the 2 operators, were correlated to clinical and serological parameters of disease activity. Statistical analysis was performed by the EPI-STAT program. Results. In SLE, synovitis was found in 21 knees (40%), joint effusion in 12 (23%), synovial proliferation in 12 (23%), positive PD signal in 5 (10%) and gastrocnemius-semimembranosus bursitis in 5 (10%). No erosions were detected. There teas a significant difference respect to RA for synovitis (p