T cells in vivo have been shown to accumulate DNA damage with age. To investigate the effects of DNA damage on T cell biology we have utilised an in vitro human CD4+ T cell clone model. Levels and types of DNA damage were determined in 11 independent T cell clones as a function of their in vitro lifespan. Increased levels of reactive oxygen species (ROS) induced DNA damage with increasing age were found in all clones analysed using a modified alkaline comet assay. T cell clones underwent apoptosis at the end of their lifespans. There were no consistent changes in the mRNA levels for the cyclin-dependent kinase inhibitors (CKI) p16, p21, and p27 during the clones' lifespans. It appears that the increased levels of ROS induced DNA damage in the T cells is not the major trigger of apoptosis, via the p53/p21 pathway. In addition, at the end of their lifespans, the T cell clones did not display the CKI phenotype reported for senescent cells (an increase in p16 and p21 levels). Thus, while the T cell clones appear sensitive to ROS-induced DNA damage, the molecular mechanisms through which this influences T cell dysfunction with age remains to be elucidated.