Circulating CD8+ T cells and monocytes are activated during human immunodeficiency virus (HIV) infection and colocalize in the aortas of simian immunodeficiency virus-infected nonhuman primates. We hypothesized that CD8+ T cells could exert a proatherosclerotic effect via paracrine actions on monocytes. We found that T-cell receptor-stimulated CD8+ T cells induce monocytes to express tissue factor, a potent activator of coagulation. Tumor necrosis factor was both necessary and sufficient for this effect.