The aim of the present study was toanalyze the potential role of serum free light chain (FLC) levels as biomarkersof disease activity in patients with systemic lupus erythematosus (SLE). Therationale behind the study is that hyperactivity of B cells is one of the mainpathogenic factors in SLE. As a result of this B cell hyperactivity, FLCs areincreased in SLE patients and may be used as predictors of flare. In fact, in aprevious study, high urinary levels of FLCs were found in periods of active SLE(J Clin Immunol 2000;20:123–137). Furthermore,they preceded the SLE flare by 4–8 weeks.