STEC-HUS is usually initiated a few days after diarrhea caused by STEC, and the shiga-toxin is central in causing endothelial cell damage, initiating the disease process.The common features for TMAs are microangiopathichemolysis,thrombocytopenia, and thrombi in small vessels leading to end organ damage. Damage to endothelial cells is the primary event in the pathogenesis of hemolytic-uremic syndrome (HUS). Endothelial injury has been attributed to multiple factors,There is evidence for complement activation during STEC-HUS,even if the condition is not primarily a complement-mediated disorder. Complement activation on RBCs could play a role in the hemolytic process occurring during STEC-HUS. Patient samples may exhibit low serum levels of C3 and subsequently elevated complement-degradation products.We presented one case , when onset and progress of illness was tipical for STECHUS , but after CNS involvement, and blood investigation results on complement, allowed us starting suitable treatment for complement mediated STEC-HUS and confirmed activation of the alternative pathway in the pathobiology of STEC-HUS.