The recovery of lymphocyte count, CD4+ and CD8+ T-cell subsets, natural killer (NK) cells and CD19+ B cells has been evaluated during the first 4 months after the infusion of autologous CD34+ peripheral blood progenitor cells (PBPC; group A; 33 patients) or autologous unselected PBPC (group B; 36 patients) for hematological malignancies. Lymphocyte count promptly recovered in both patient cohorts, although the repopulation of CD3+ T cells occurred more rapidly in group B compared with group A. The count of CD4+ T lymphocytes remained <200/µl during the study period in patients transplanted with CD34+ PBPC, being significantly lower compared with group B (p = 0.0019 and p = 0.0035 on days 30 and 60, respectively). CD8+ T cells rapidly increased both in group A and B and CD4 to CD8 ratio was severely reduced. CD4+ and CD8+ T cells displayed an activated phenotype in both groups of patients, coexpressing the HLA-DR antigen throughout the study period. No differences in the repopulation kinetics of NK cells and CD19+ B cells were observed. Further investigations are encouraged to characterize T cell competence following transplantation of CD34+ PBPC.