Gliostatin/platelet-derived endothelial cell growth factor (GLS/PD-ECGF) has a potential for arthritogenic action. The aim of this study was to examine whether measurement of serum GLS can be used to evaluate symptomatic improvements after surgery (arthroplasty or synovectomy) as well as the aggressiveness of disease activity in rheumatoid arthritis (RA). Serum GLS levels were determined by enzyme immunoassay in rheumatoid factor (RF)-positive and -negative RA patients. In those undergoing surgery, levels were measured 3 months before and after the operations. Both RF-positive and -negative RA sera showed higher GLS levels than normal and osteoarthritis sera. Patients undergoing arthroplasty demonstrated a decrease in serum GLS levels after the operations, but patients undergoing synovectomy did not, reflecting the extent of remaining or reproliferating synovial tissues rich in GLS production. These findings suggest that the serum GLS level is a useful indicator for evaluation of synovitis and the systemic efficacy of surgical treatment.