The thymus represents the major site of lymphopoiesis of T-cell receptor (TCR) αβ T-cells. Age-related involution may affect its potential to reconstitute T-cells that are lost during HIV infection, chemotherapy, and bone marrow transplantation. However, there is mounting evidence that the age-related changes in the thymus are quantitative, not qualitative, and recent data suggest that the adult thymus can indeed contribute to T-cell reconstitution. Using newer methods to assess thymic function, it can be shown that the increases in naı̈ve T-cell numbers in patients receiving antiretroviral therapy for AIDS are largely derived from the thymus. This provides direct evidence for the functional capacity of the adult thymus.