The use of intravenous tissue plasminogen activator (tPA) in the treatment of ischemic stroke is limited by its propensity to exacerbate brain edema and hemorrhage. The mechanisms underlying these deleterious effects of tPA remain incompletely understood. The purpose of this study was to delineate a pathway of tPA-mediated complement cascade activation in stroke and to determine whether complement inhibition ameliorates the adverse effects of post-ischemic tPA administration. We found that tPA promotes C3 cleavage both in vitro and in ischemic brain through a plasmin-mediated extrinsic pathway. Using cell culture models, we then showed that the C3a-receptor is strongly expressed on ischemic endothelium and that exogenous C3a dramatically enhances endothelial cell permeability. Next, we assessed the effect of tPA administration on brain edema and hemorrhage in a transient model of focal cerebral ischemia in C57BL/6 mice. We found that intravenous tPA exacerbates brain edema and hemorrhage in stroke, and that these effects are abrogated by a small-molecule antagonist of the C3a receptor. These findings establish for the first time that intravenous tPA dramatically upregulates complement cascade activation in ischemic brain and that pharmacologic complement inhibition protects against the adverse effects of tPA-mediated thrombolysis in stroke.