The immunoreconstitution of blood lymphocyte subpopulations was evaluated in 44 HIV-infected children enrolled in a phase I/II clinical trial of combination zidovudine, didanosine, and ritonavir therapy. At entry, the cohort had a median age of 6.8 years, advanced disease (57% CDC stage C, 73% immune stage 3), and was naive to protease inhibitor therapy. The median decrease in viral load was 1.7 logs by 4 weeks of therapy, and remained low for the 24 weeks of observation in the majority of subjects. However viral burden returned to baseline levels by 8 weeks in nearly a third of patients. FACS analysis of peripheral blood lymphocyte populations revealed a significant increase in CD4+ and CD8 + T cells after 4 weeks of therapy (mean increases of 266 +/- 178 cells/ul and 597 +/- 661 cells/ul. respectively, p<0.05). CD4+ T cells continued to increase over the 24 weeks while CD8+ T cells returned to baseline by 12 weeks. B cells demonstrated a sustained increase during therapy while NK cells showed a transient increase. Increases were initially observed in CD4+ CD45RA T cells. There were significant fluctuations in median CD45RA/CD45RO T cell ratio from 1.1 at entry to 0.6 at 4 weeks, to 1.4 at 24 weeks. A mean rise in CD4+ T cell numbers of 224 +/- 60 cells occurred by 24 weeks, even in patients whose viral burden returned to baseline levels by 8 weeks. Multi variant analysis revealed that children under 6 years of age had a higher degree of reconstitution in all lymphocyte populations compared to older children. The dynamics of immune reconstitution following ritonavir therapy is characterized by significant lymphopoiesis within B cell and T cell populations with considerable potential for reconstitution of thymic dereived CD4+CD45RA T cells in HIV-infected children. Improved CD4+ T cell numbers was evident in children with advanced disease in spite of a lack of decrease in viral burden with therapy.