Severe deficiency of plasma ADAMTS13 activity is the primary cause of thrombotic thrombocytopenic purpura (TTP) whereas overwhelming activation of complement via an alternative pathway results in atypical hemolytic uremic syndrome (aHUS), the prototypes of thrombotic microangiopathy (TMA). However, clinical and pathogenic distinctions between TTP and aHUS are often quite challenging. Clinical reports have suggested that complement activation may play a role in the development of TTP, which is caused by severe deficiency of plasma ADAMTS13 activity. However, the experimental evidence to support this hypothesis is still lacking. Here, we show that mice with either Adamts13−/− or a heterozygous mutation of complement factor H (cfh) at amino acid residue of 1206 (ie, cfhW/R) alone remain asymptomatic despite the presence of occasional microvascular thrombi in various organ tissues. However, mice carrying both Adamts13−/− and cfhW/R exhibit thrombocytopenia, low haptoglobin, increased fragmentation of erythrocytes in peripheral blood smear, increased plasma levels of lactate dehydrogenase activity, blood urea nitrogen, and creatinine, as well as an increased mortality rate, consistent with the development of TMA. Moreover, mice with a homozygous mutation of cfh (ie, cfhR/R) with or without Adamts13−/− developed severe TMA. The mortality rate in mice with Adamts13−/−cfhR/R was significantly higher than that in mice with cfhR/R alone. Histological and immunohistochemical analyses demonstrated the presence of disseminated platelet-rich thrombi in terminal arterioles and capillaries of major organ tissues in these mice that were either euthanized or died. Together, our results support a synergistic effect of severe ADAMTS13 deficiency and complement activation in pathogenesis of TMA in mice. •Mice with Adamts13−/− or a heterozygous cfh mutation (ie, chfW1206R) do not develop spontaneous TMA.•However, mice carrying Adamts13−/−cfhW1206R or cfhR1206R develop TMA with a significantly increased mortality rate. Display omitted