ADAMTS13 is a protease that cleaves plasma VWF which will prevent the microthrombus formation caused by UL-VWF. The accumulation of UL-VWF can be caused by congenital or acquired deficiency of ADAMTS13 in patients with Thrombotic thrombocytopenic purpura (TTP) which is characterized by thrombocytopenia, microangiopathic hemolytic anemia with schistocytes, variable neurologic and renal symptoms, and fever. ADAMTS13 activity and inhibitor levels in patients diagnosed as TTP and receiving regular therapeutic plasma exchange (TPE) sessions were assessed using ELISA technique. This study was carried out on 24 patients diagnosed as thrombotic thrombocytopenic purpura: 8 males and 16 females. Their age ranged between 16 and 60 years, of them, 3 cases were known as IV drug addicts and 1 female was pregnant 32 weeks. Twenty geographically and racially matched healthy controls, 8 males and 12 females were included in the study. Samples were drawn before starting and at the end of the sessions to evaluate the role of ADAMTS13 activity and anti-ADAMTS13 antibodies in the pathogenesis and prognosis of TTP patients. 11 out of 24 cases have recovered while 13 cases died. Plasma exchange led to rise in ADAMTS13 activity level in 11 patients with average 14 plasma exchange sessions. 3 cases were known to be IV drug addicts where all of them were males and diagnosed as secondary TTP.