Acquired immune responses mediated by CD4(+) T cells contribute to the initiation and progression of acute coronary syndrome (ACS). ACS patients show acquired immune system abnormalities that resemble the characteristics of autoimmune dysfunction described in the elderly. This study aimed to investigate the role of premature CD4(+) T cells senescence in ACS and the underlying mechanism. We compared the immunological status of 25 ACS patients, 15 young healthy individuals (C1), and 20 elderly individuals with absence of ACS (C2). The percentages of CD4(+) T lymphocyte subsets (including naive, regulatory, memory and effector T cells) in peripheral blood were analyzed. In ACS patients, a significant expansion of CD4(+)CD28(null) effector T cells and a decline of CD4(+)CD25(+)CD62L(+) Treg cells were observed. In addition, patients with ACS showed an accelerated loss of CD4(+)CD45RA(+)CD62L(+) naive T cells and a compensatory increase in the number of CD4(+)CD45RO(+) memory T cells. ACS patients demonstrated no significant difference in frequency of T cell receptor excision circles (TRECs) compared to age-matched healthy volunteers. The expression of p16(Ink4a) was increased while CD62L was decreased in CD4(+)CD28(null) T cells of ACS patients. Compared to healthy donors, ACS patients demonstrated the lowest telomerase activity in both CD4(+)CD28(+) and CD4(+)CD28(null) T cells. The serum levels of C-reactive protein, Cytomegalovirus IgG, Helicobactor pylori IgG and Chlamydia pneumonia IgG were significantly higher in ACS patients. The results suggested that the percentage of CD4(+) T cell subpopulations correlated with chronic infection, which contributes to immunosenescence. In conclusion, chronic infection induced senescence of premature CD4(+) T cells, which may be responsible for the development of ACS.