To evaluate the ability of ultrasonography with high frequency transducer (13-MHz) in detecting metacarpophalangeal (MCP) joint abnormalities, 20 patients with rheumatoid arthritis (RA) and a symptomatic involvement of MCP joints were studied. Twenty healthy subjects served as controls. In all RA patients, ultrasonography clearly demonstrated one or more soft tissue and/or bone abnormalities. Sixteen patients (80%) had joint cavity widening because of effusion (1 case), synovial thickening (7 cases), and joint effusion with synovial thickening (8 cases). Loss of definition of the metacarpal articular cartilage was observed in 17 cases (85%). Sixteen (80%) had bone erosions and 9 (45%) showed a definite widening of the flexor tendons sheath. Margin irregularities of the extensor and flexor tendons were respectively visualized in 7 (35%) and in 8 (40%) cases. An extensor tendon rupture was observed in 2 cases (10%). Ultrasound examination by 13-MHz transducer appears to increase the accuracy of MCP joints evaluation in RA with respect to conventional radiography. It can be recommended as the first investigation in the assessment of soft tissue involvement in RA patients.