OBJECTIVE: Rheumatoid arthritis (RA) is a chronic, recurrent, systemic inflammatory disease and results in major deformity or dysfunction of joints. A disease activity parameter has important clinical significance because it is a useful objective tool for assessing disease activity and planning individualized therapeutic program. However, in RA, only a few parameters have been available such as C-reactive protein (CRP), erythrocyte sedimentation rate (ESR), or rheumatoid factor (RF). By using these parameters, it is difficult to recognize concomitant nephropathy in RA. Therefore, as a new disease activity parameter for RA and a marker for nephropathy, we evaluated polymerized C9 which is the neoantigen produced after in vitro activation of complement and is quantified by using an enzyme conjugated monoclonal antibody. METHODS: Polymerized C9 (CAE, INCSTAR-DiaSorin, Italy), CRP, ESR and RF testing on 86 patients diagnosed with RA were undertaken. According to clinical and laboratory review, we grouped the patients into three disease categories as active RA (symptomatic, n=43), inactive RA (asymptomatic, n=35), and RA with nephropathy (n=8) and compared the means between three groups. RESULTS: In comparing with each disease monitoring parameter between groups, polymerized C9, CRP, ESR, and RF discriminated active RA from inactive RA (p<0.05). The ROC performance test showed polymerized C9 and CRP are the two best parameters in discriminating disease activity of RA. Furthermore, only polymerized C9 accurately discriminated disease activity and also predicted nephropathy in RA patient (p<0.05). CONCLUSION: Like CRP, Polymerized C9 can be a useful disease activity parameter for RA and also a predictive parameter for RA nephropathy.