Introduction: Lupus nephritis occurs in over 50% of patients with systemic lupus erythematosus (SLE). Reports of 5-year renal survival with treatment ranges from 46 to 95%. Early diagnosis and prompt treatment, however, may significantly improve the long-term prognosis. Lupus nephritis (LN) is a major cause of morbidity and mortality in systemic lupus erythematosus (SLE). As the course of LN is often unpredictable, it is important to identify reliable, noninvasive methods to repeatedly assess the condition of the kidneys in these patients. Urinary biomarkers are easily obtained and probably are best at reflecting the current renal status, as they specifically represent local inflammatory activity. Renal biopsy is the “gold standard” to determine renal activity in systemic lupus erythematosus (SLE), but it is expensive, invasive, and carries risk. Osteoprotegerin (OPG), a member of the tumor necrosis factor (TNF) receptor family, has been identified as a regulator of bone resorption. It has been demonstrated that OPG is produced by a variety of organs and tissues, including the cardiovascular system (heart, arteries, veins), lung, kidney, and immune tissues, as well as bone. It is hypothesized that kidney excretion plays an important role in the clearance of OPG, Thus OPG concentration in the urine might rise in a lupus nephritis flare, because of the increased production and excretion from inflamed microvascular endothelial cells in the kidney. Aim Of The Work: To study urine OPG as a potential biomarker for lupus nephritis activity. Material And Methods: The SLE patients divided into: Renal disease group (based on results of a kidney biopsy demonstrating immune complex-mediated glomerulonephritis, as well as evidence of major renal manifestations past or present attributable to SLE, such as proteinuria and/or elevated serum creatinine). Non-Renal disease group. 40 patients will participate in this study; 25 SLE patients with renal affection as proved by renal biopsy and 15 SLE patients without protinuria. Results: There is statistically high significant between both groups regarding urine OPG level. Also there is statistically high significant between urine OPG level and serum creatinne level as well as urine protein/creatinine ratio in whole patients. Also there is statistically high inverse significant between urine OPG level and haemoglobin level as well as inverse significant relation between urine OPG level and serum complements 3 level.