Parasite burden predicts disease severity in malaria and risk of death in cerebral malaria patients. In murine experimental cerebral malaria (ECM), parasite burden and CD8+ T cells promote disease by mechanisms that are not fully understood. We found that the majority of brain-recruited CD8 + T cells expressed granzyme B (GzmB). Furthermore, gzmB -/- mice harbored reduced parasite numbers in the brain as a consequence of enhanced antiparasitic CD4+ T cell responses and were protected from ECM. We showed in these ECM-resistant mice that adoptively transferred, Ag-specific CD8+ T cells migrated to the brain, but did not induce ECM until a critical Ag threshold was reached. ECM induction was exquisitely dependent on Ag-specific CD8+ T cell-derived perforin and GzmB, but not IFN-γ. In wild-type mice, full activation of brain-recruited CD8+ T cells also depended on a critical number of parasites in this tissue, which in turn, was sustained by these tissue-recruited cells. Thus, an interdependent relationship between parasite burden and CD8+ T cells dictates the onset of perforin/GzmB-mediated ECM.