To the Editor: Mackall et al. (Jan. 19 issue)1 clearly demonstrate an inverse relation between age and the ability to regenerate CD4+ populations of T lymphocytes after intensive chemotherapy. They suggest that involution of the thymus (which occurs naturally with age) is likely to account for the diminished capacity to produce CD4+ cells. In the light of what is now known about extrathymic T-cell differentiation, however, the conclusion that “rapid T-cell regeneration requires residual thymic function” cannot be made without first examining CD8+ cell populations, which may have differentiated without a thymic influence. Studies in mice have shown that T-lymphocyte. © 1995, Massachusetts Medical Society. All rights reserved.