Systemic lupus erythematosus (SLE) is a prototype of human systemic autoimmune diseases. The pathogenesis behind the disease remains unclear. It is a chronic disease characterized by multisystem organ affection; with recurrent remissions and exacerbations. Therefore, detecting changes in disease activity is of great importance. B lymphocyte stimulator (BLyS) is a noval member of the tumor necrosis factor (TNF) ligand superfamily. It is a potent B cell survival factor, inducing B cell proliferation, differentiation, and immune modulation. Now, there are growing evidences that BLyS contributes to the pathogenesis of SLE.Aim of the work:The aim of this work is to study the expression of the BLyS mRNA using reverse transcription- polymerase chain reaction (RT-PCR) to examine the hypothesis that enhanced expression of BLyS gene is associated with increased disease activity. Results: Overexpression of BLyS gene was significantly correlated with the disease activity.