Background Elevation of free light chains (FLC) represents B cell activation and has been detected in the autoantibodies positive rheumatologic disorders such as rheumatoid arthritis (RA) and Sjögren syndrome. Psoriatic arthritis (PsA) is typically not accompanied by autoantibodies, however, B cells have been found in PsA synovium (Ref1). Objectives We have conducted a case control study to evaluate the frequency of elevated serum FLC free kappa and free lamda levels in 210 PsA patients to detect polyclonal or monoclonal aberration. Methods Serum FLC levels were measured using the Freelite immunoassay (The Binding Site, Germany) to detect free kappa (normal range 3.3-19.4 mg/l) and lamda (5.7-26.3). Increased free kappa and free lamda levels were defined as polyclonal, a kappa/lamda ratio <0.26 or >1.65 was defined as monoclonal aberration. As controls served healthy controls (n=50), RA patients (n=75) and B-NHLymphoma (n=288). A subgroup analysis was performed after stratification of PsA patients into age, disease duration, development of joint erosions, biological treatment, elevated TNF alpha serum levels. Results PsA patients showed significantly less frequently polyclonal respectively monoclonal increase of FLC (7.1 and 3.8%) compared to RA (10.7 and 10.7%, **p=0.001) and B-NHLymphoma (11.5 and 10.1%, ***p=0.001), but more than in healthy controls (*p=0.001, figure 1). PsA patients with increased TNF alpha serum levels and patients with erosive joint disease had significantly more frequently polyclonal aberrations. Image/graph Conclusions Subgroups of PsA patients showed polyclonal aberrations measured by serum FLC indicating B cell activation in the course of disease. References Cantete JD, et al. Ectopic lymphoid neogenesis in psoriatic arthritis. ARD 2007;66,720-26 Disclosure of Interest G. Assmann Grant/research support from: THE BINDING SITE, Schwetzingen, Germany, put the FREELITE immunoassay at costfree disposal for G Assmann, S. Zinke: None Declared, J. T. Ney: None Declared, M. F. Ong: None Declared, F. Neumann: None Declared, S. Nold: None Declared, M. Pfreundschuh: None Declared