The long-term immunologic effects of intermittent interleukin 2 (IL-2) therapy were evaluated in a cross-sectional study by comparing 3 groups: HIV-seronegative volunteers, HIV-infected (HIV⁺) patients receiving highly active antiretroviral therapy (HAART), and HIV⁺ patients receiving HAART and intermittent IL-2. Whole-blood immunophenotyping was performed to study expression of the IL-2 receptor chains on T lymphocytes and natural killer cells and to further characterize CD4⁺/CD25⁺ T cells. Increased CD25 expression, especially in CD4⁺ T cells but also in CD8⁺ T cells, without increases in expression of the β and γ chains of the IL-2 receptor was detected in the IL-2 group. Up to 79% of naive CD4⁺ T cells (median, 61%) from patients in the IL-2 group expressed CD25, and the number of naive CD4⁺/CD25⁺ T cells correlated positively with both the total and naive CD4⁺ T-cell counts. A discrete population of CD45 double intermediate RA⁺/RO⁺CD4⁺ cells was also preferentially expanded in the IL-2 group, and the number of these cells strongly correlated with the total CD4⁺ count. Despite increases in CD25 expression, T lymphocytes from patients treated with IL-2 did not have increased expression of early (CD69) or late (CD95) activation markers or evidence of recent proliferation (Ki67). Both CD4⁺/CD25⁺ and CD4⁺/CD25⁻ cells from IL-2–treated HIV⁺ patients proliferated in response to mitogens, specific antigens, and T-cell-receptor–mediated stimuli. Thus, intermittent administration of IL-2 in HIV⁺ patients leads to preferential expansion of a unique subset of CD4⁺ T cells that may represent a critical population in T-cell homeostasis.