Although the cause of ANCA-associated vasculitis (AAV) remains undetermined, the presence of lymphocytic infiltrates in inflammatory lesions of patients suggests that vascular damage is immune mediated. Studies over the past decade have implicated a role for T cells in the pathogenesis of AAV as altered T cell phenotype has been observed in this disorder. The distribution of T cell subpopulations has been analyzed most intensely in Wegener's granulomatosis (WG), where an expanded population of circulating CD4(+) effector memory T cells (CD4(+)T(EM)) was demonstrated. CD4(+)T(EM) cells play a major role in the pathogenesis of several autoimmune diseases. Specific suppression of CD4(+)T(EM) cells inhibits delayed-type hypersensitivity (DTH) and has therapeutic potential in autoimmune disease. Thus, CD4(+)T(EM) cells may act as inducers of tissue injury and participate in the development of AAV. Therapies that target CD4(+)T(EM), without impairing the activity of other lymphocyte subsets, may hold therapeutic promise for AAV.