The sub-laboratory effects of SLE on blood cells were not completely evaluated to explain the thrombotic tendency found in this disease. The present study was conducted to assess the ultrastructural changes of RBCs, WBCs and platelets in SLE and to corelate these changes with the disease activity. Comet assay and 8hydroxydeoguanosine (8-OHdG) were used to confirm cellular dysfunction. Ninety subjects were recruited and equally divided into 3 groups: Group Ι; SLE (normal CBC), Group ΙΙ; SLE (abnormal CBC) and Group ΙΙΙ; healthy control. Disease activity was evaluated by systemic lupus erythematosus disease activity index score (SLEDAI). Ultrastructure examination of blood cells were done by electron microscope, DNA damage was assessed by Comet assay and serum 8-OHdG levels. CBC and serological tests including serum C3, C4, ANA and anti-dsDNA were evaluated. There was statistically significant negative correlation between RBCs and WBCs elements with SLEDAI score in Group II. There was significant statistical difference in RBCs and WBCs cell membrane defects by electron microscope between Group I and Group II. There was no statistically significant correlation between blood cell membrane defects and SLEDAI score in both Group I and Group II. There was a significant increase in percentage of tail DNA damage (p |