During acute human viral infections, such as influenza A, specific cytotoxic T lymphocytes (CTL) are generated which aid virus clearance. We have observed that in HLA-A*0201⁺ subjects, CTL expressing V_β17⁺ TCR and recognizing a peptide from the influenza A matrix protein (M1_58–66) dominate this response. In experimental models of infection such dominance can be due to inheritance of a restricted T cell repertoire or acquired consequent on expansion of CTL bearing an optimum TCR conformation against the MHC–peptide complex. To examine how influenza A infection might influence the development of TCR V0_β17 expansion, we studied influenza A-specific CTL in a cross-sectional study of 82 HLA-A*0201⁺ individuals from birth (cord blood) to adulthood. Primary M1_58–66 -specific CTL were detected in cord blood, but their TCR were diverse and depletion of V_β17⁺ cells did not abrogate specific cytotoxicity. In contrast following natural influenza A infection, TCR V_β17⁺ CTL dominated to the extent that only one of nine adult CTL lines retained any functional activity after in vitro depletion of V_β17⁺ CTL. These results suggest that the dominance of V_β17⁺ TCR among adult M1_58–66-specific CTL results from maturation and focussing of the response driven by exposure to influenza, and have implications for optimum immunization strategies.