The aims of our study were to investigate the prevalence of ultrasound (US) pathologic abnormalities and to compare them with the clinical findings in the knee of rheumatoid arthritis (RA) patients. One hundred RA patients were enrolled in the study. Bilateral US examination of the knee was performed to visualise the presence of effusion, synovial proliferation, bone erosions, femoral cartilage abnormalities, quadricipital and/or patellar enthesopathy. The popliteal fossa and the calf region were also evacuate to detect popliteal cyst. We observed joint effusion in 140 out of 200 (70%) knees. Synovial hypertrophy was present in 115 out of 140 (82%) knees associated with effusion and in 22 out of 115 (19%) knees intra-articular power Doppler (PD) signal was found. Hyperechoic spots within the cartilage layer, suggestive of pyrophosphate crystals deposit, were detected in the knees of 3 patients. US signs of quadricipital and/or patellar enthesopathy were detected in 53 out 200 (26%) knees. Bone erosions were visualised in 16 out 200 (8%) knees. Popliteal cyst was found in 66 out of 200 (33%) joints. US examination of the knee is more sensitive than clinical examination in the detection of joint inflammation and allows for the identification of different patterns of pathologic changes at knee level, including morphostructural changes at both cartilage and tendon level.