Effects of GH on thymic output and T cell proliferation have been described. Combined interpretation of T cell and TREC dynamics can be used to determine whether these effects contribute to increased naive CD4⁺ T cell numbers after rGH treatment in HIV-1–infected individuals. The figure exemplifies T cell dynamics (percentage of naive CD4⁺ T cells within the peripheral PBMC pool and number of naive CD4⁺ T cells) and TREC dynamics (TREC numbers and percentage of TREC-containing cells within the naive CD4⁺ T cell and PBMC population) prior to GH treatment (A) and different possible scenarios following GH treatment (B–D). Prior to GH treatment (A), the thymic production of naive CD4⁺ T cells shown is 2, and these cells divide 1 time in the periphery. This results in 4 naive CD4⁺ T cells (gray), which in total contain 2 TRECs. In scenario B, rGH is shown to only affect thymic output (4 naive CD4⁺ T cells). In scenario C, rGH is shown to only affect peripheral CD4⁺ T cell production (these cells are shown to divide twice in the periphery). In their current study in this issue of the JCI, Napolitano et al. (13) show that in fact rGH administration to HAART-treated HIV-1–infected adults concurrently affects both thymic output and peripheral naive CD4⁺ T cell proliferation, as shown in scenario D. The resultant effects include increased thymic production of naive CD4⁺ T cells and TRECs, an increase in the percentage of naive CD4⁺ T cells and TRECs within the peripheral PBMC pool, but decreased TREC frequency within naive CD4⁺ T cells.