Previous studies have shown a slow recovery of naive CD4+ T cell counts after anti-retroviral therapy in HIV-1-infected adults, which is in accordance with thymus atrophy after puberty. Here we investigate whether or not different patterns of naive CD4+ and CD8+ T cell repopulation are present in adult and child patients undergoing anti-retroviral treatment. Thus, 25 adults under highly active anti-retroviral therapy and 10 children under combined anti-retroviral therapy were retrospectively analysed for T cell subpopulations at baseline (T0) and around week 12 (T1) and week 24 (T2) of anti-retroviral treatment. Mean serum HIV-1 RNA levels dropped in both groups. Recovery of T cells in adults was characterized by a heterogeneous response between patients, with only 44% of them increasing their naive CD4+ and CD8+ T cell counts at T1, and changes in mean total CD4+ T cells were mainly shaped by memory cells. Otherwise, children were characterized by an early increase in naive T cells. Thus, at T1, all children analysed had a strong rise in CD4+ (from 389 +/- 116 to 569 +/- 121 cells/microl; P < 0.01), and nine out of 10 also in naive CD8+ T cells (from 244 +/- 58 to 473 +/- 85 cells/microl; P < 0.05). However, no significant correlation between age and naive repopulation was observed (P = 0. 22) in children. Thus, children had the earlier and greater increases in naive T cell subsets than adults, probably due to a more active thymus, with the potential for immune reconstitution when HIV-1 replication is controlled.