(1) C1q, C3, and C5 deposited in the ischemic side of WT but not contralaterally. (2) C1q−/− did not experience improvement in cerebral infarct volumes, neurological outcome, or mortality compared to WT. (3) C3−/− and C3aRA-treated WT had significant improvement in infarct volume and neurological outcome with less granulocyte infiltration and no effect on mortality. (4) C3−/− improvement was reversed in C3-treated C3−/− mice. (5) C5−/− did not have significant change in infarct volume or mortality. Outcomes were assessed 24 h after ischemia. The gene discussed is C3; the disease is cerebral infarction.