Studies on persistent viral infections demonstrate that CD8(+) T-cells differentiate along distinct pathways following chronic antigen exposure; however the effect of stimulation with non-viral chronic antigens is poorly described. We assessed the contributions that the presence of an allograft or cytomegalovirus (CMV) has on the post-thymic differentiation of CD8(+) T-cells in both the blood and lung allograft in patients undergoing lung transplantation. CD28 expression on blood CD8(+) T-cells was reduced in CMV seropositive patients, and was further reduced following acute episodes of CMV reactivation. These viral-associated changes in phenotype were not seen in CD8(+) T-cells isolated from the lung allograft where a different pattern of CD28 expression was observed. Following transplantation there was a progressive reduction in CD28 expression on BAL CD8(+) T-cells. In contrast to what was observed in peripheral blood, reduced CD28 expression on BAL CD8(+) T-cells was associated with a reduced gamma-IFN production. Furthermore, a high proportion of CD28-CD8(+) T-cells in the BAL was associated with fewer episodes of acute allograft rejection. An expanded CD28-CD8(+) T-cell subset, with reduced function, within the lung allograft may have important prognostic implications in lung transplant recipients.