Clinical trials and animal studies have revealed a role for the renin-angiotensin system in the enhanced thrombus development that is associated with hypertension. Because T lymphocytes have been implicated in the vascular dysfunction and blood pressure elevation associated with increased angiotensin II (Ang II) levels, we evaluated the role of the adaptive immune system in mediating the enhanced thrombosis during Ang II–induced hypertension. Light/dye-induced thrombosis was induced in cremaster arterioles of wild-type, immunodeficient Rag-1 −/− , CD8 + , or CD4 + lymphocyte-deficient and NADPH oxidase (gp91 phox )–deficient mice implanted with an Ang II–loaded pump for 2 weeks. Chronic Ang II infusion enhanced arteriolar thrombosis in wild-type mice but not in Rag-1 −/− , CD4 + T-cell–deficient, or gp91 phox−/− mice. CD8 + T-cell −/− mice exhibited partial protection. Adoptive transfer of T cells derived from wild-type or gp91 phox−/− mice into Rag-1 −/− restored the prothrombotic phenotype induced by Ang II. T lymphocytes (CD4 + and, to a lesser extent, CD8 + ) play a major role in mediating the accelerated microvascular thrombosis associated with Ang II–induced hypertension. NADPH oxidase–derived reactive oxygen species, produced by cells other than T lymphocytes, also appear critical for the Ang II–enhanced, T cell–dependent thrombosis response.