Systemic lupus erythematosus is a serious multisystem disease characterized by complex alterations of the immune system and an immune complex vasculitis with endothelial cell damage.Thrombomodulin is a transmembrane glycoprotein receptor of thrombin, recently established as a marker of endothelial cell damage. Plasma thrombomodulin are increased in a certain number of pathologies associated with endothelial lesions including SLE and its plasma levels are related to the severity of the pathology. The aim of the present work is to study plasma TM level in patients with SLE and correlate its level with disease activity. This study included fifty SLE patients, satisfying at least four criteria of ARA classification of SLE, selected from the outpatient rheumatology clinic in Cairo University Children’s Hospital.The results showed that:. The mean plasma TM levels were significantly higher in SLE patients than in healthy controls, in active disease than inactive disease, and correlated positively with high ESR values and low C3.In conclusion, the data support the role of TM as a relatively good marker of disease activity in SLE. This molecule is thus suitable for the monitoring of disease activity, to predict flare and vascular complications as well as to monitor the therapeutic response.