Systemic lupus erythmatosus (SLE) is a chronic potentially fatal autoimmune disease characterized by unpredictable exacerbation and remissions with protean clinical manifestations. The aetiology of SLE is not known. It can occur in all ages, but more common in young women of child bearing age. The health status of a SLE patient is related mostly to disease activity; damage that results from recurrent episodes of disease flare, as well as the adverse effect of the treatment severely affects the general health, thus, assessment of the disease activity is one of the important screening studies for proper treatment avoiding early complications.Particular attention has been focused on neopterin as an indicator of activation of the cellular immune system. This case-control study was conducted in Kasr El-Aini hospital, Cairo University. Thirty patients with systemic lupus erythematosus (SLE) were selected from the outpatient section of the internal medicine department. Age and sex matched controls were selected from volunteer employees who did not complain of any rheumatic diseases.