BACKGROUND Infiltration by perforin-secreting killer lymphocytes, such as T cells and natural killer cells, has been shown to be involved in the pathogenesis of vascular cell damage in Takayasu's arteritis. RESULTS To investigate the immunological mechanisms involved, especially the nature of T-cell infiltration in Takayasu's arteritis as well as atherosclerosis, we analyzed the expression of T-cell receptor (TCR) Valpha and Vbeta genes in infiltrating cells in the aortic tissue of patients with Takayasu's arteritis and the atherosclerotic aortic aneurysm by polymerase chain reaction (PCR). We also analyzed the expression of cytokine genes by PCR. We found that the repertoires of TCR Valpha as well as Vbeta gene transcripts in Takayasu's arteritis were restricted. The infiltrating cells expressing Valpha2, Valpha16, Valpha17, Vbeta7, and Vbeta13.1 were found in 3 of 4 patients. In contrast, TCR Valpha-Vbeta repertoires in atherosclerotic aortic aneurysm were polyclonal. There was no significant difference in the pattern of cytokine gene expression between the two diseases. CONCLUSIONS The restricted usage of TCR Valpha as well as Vbeta genes by infiltrating T cells in Takayasu's arteritis may indicate that a specific antigen in the aortic tissue was targeted. Our findings provide the evidence that distinct immunological mechanisms are involved in the pathogenesis of Takayasu's arteritis and atherosclerotic aortic aneurysm.