To examine the impact of thymic size on immune recovery in patients with human immunodeficiency virus (HIV) infection, the thymus was visualized, using computed tomographic scans, in 25 HIV-infected patients who had received highly active antiretroviral therapy (HAART) for 6–18 months and had levels of viremia <500 copies/mL. For comparison, 10 control subjects were included in the study. Total and naive CD4⁺ cell counts were determined by flow cytometry. To determine thymic output, the number of CD4⁺ cells containing T cell receptor excision circles (TRECs) was measured. Qualitative immune recovery was evaluated by determination of CD4⁺ T cell receptor repertoire in 19 of the HIV-infected patients. Larger thymic size was associated with higher CD4⁺ cell counts (r = 0.498; P = .011) and higher CD4⁺ TREC frequency (r = 0.652; P <.001). Furthermore, patients with abundant thymic tissue seemed to have broader immunologic repertoires, compared with patients with minimal thymic tissue (P = .054). These findings suggest that thymopoiesis is ongoing in the adult thymus and contributes to immune reconstitution in HIV-infected patients receiving HAART.