Purpose : HUS usually occurs in children after infection with shiga toxin-producing microorganism(D＋HUS). In contrast, non-postdiarrheal(D-) HUS occurs at any age and has a high rate of relapse and a poor prognosis. The clinical presentation of D-HUS is similar to that of thrombotic thrombocytopenic purpura(TTP). Recently severe deficiencies of ADAMTS13 were reported not only in TTP and D- HUS but also in D+ HUS during their acute phase. The purpose of the study is to evaluate the plasma ADAMTS13 activity in D＋ and D-HUS. Methods : Nineteen children with HUS(D＋ HUS 12 and D- HUS 7) were enrolled. The assays of plasma ADAMTS13 activity were performed during the acute stage in the D＋ HUS and at various stages of relapsing courses in the D- HUS patients by multimer assay, based on electrophoresis. Results : The median plasma activity of ADAMTS13 in D＋ HUS and D- HUS were 80.9%(37.8-132.4%) and 53.9%(1.0-94.1%), respectively, which were not statistically significantly different from control(86.4%, 34.2-112.3%)(P>0.05). One boy with D- HUS had severe deficiency of ADAMTS13(1.0%). His platelet count was normalized temporarily by fresh frozen plasma infusion. Conclusion : We have demonstrated that there is no significant difference of the plasma ADAMTS13 activity between D＋ HUS, D- HUS and control. We detected severe deficiency of ADAMTS13 in one boy who presented with relapsing episodes of D- HUS. ADAMTS13 deficiency should be considered in the subgroup of D- HUS especially with early onset and recurrent courses. Plasma therapy can be beneficial in this subgroup.