Athletes appear to be at a greater risk of illness while undertaking arduous training regimens in preparation for endurance events. As infection susceptibility has been linked with increased proportions of differentiated and senescent T cells in the periphery, changes in the proportions of these cell types due to long-term high-volume exercise training could have important implications for athlete infection risk. This study examined the effects of 6-month training preparation for an Ironman triathlon on the proportions of naïve, memory and senescent T cells in resting blood. Ten club-level triathletes (9 males; 1 female: 43 ± 3 years) were sampled at 27 (December), 21 (January), 15 (March), 9 (May) and 3 (June) weeks before an Ironman Triathlon. An additional sample was collected 2-week post-competition (August). Four-colour flow cytometry was used for the phenotypic analysis of CD4+ and CD8+ blood T cells. Proportions of differentiated (KLRG1+/CD57-) CD8+ T cells and "transitional" (CD45RA+/CD45RO+) CD4+ and CD8+ T cells increased with training, as the values in June were elevated 37, 142 and 116%, respectively, from those observed in December. Proportions of senescent (KLRG1+/CD57+) CD4+ or CD8+ T cells did not change during the training phase. Two weeks post-race, proportions of differentiated CD8+ T cells had returned to baseline values, while the proportions of senescent CD4+ T cells increased 192% alongside a 31% reduction in naïve (CD45RA+/CD45RO-) cells. In conclusion, increases in differentiated and "transitional" T cells due to arduous exercise training could compromise host protection to novel pathogens and increase athlete infection risk, although whether or not the composition of naïve and differentiated T cells in blood can serve as prognostic biomarkers in athletes remains to be established.