Immune reconstitution after marrow transplantation has some characteristics of immune development in early life. Here we provide phenotypic data suggesting this may not be true for T cells (particularly CD4+ T cells) in the case of marrow transplantation into adults. T cells from 35 adult patients at 1 year after transplant, 14 normal neonates and 22 normal adults were studied by 3-color flow cytometry. Marked disparity between the phenotype of neonatal vs post-transplant T cells was found. Of the total CD4+ T cells, the neonates had supranormal percentages of CD45RAhigh, L-selectin+, CD29low/-, CD11alow/-and CD28+ cells whereas most patients at 1 year after transplant had subnormal percentages of these CD4+ T cell subpopulations.(sub/supra-normal denotes below/above normal adult values). Absolute blood counts of naive (CD45RAhigh, L-selectin+, CD29low/-and CD11alow/-) CD4+ T cells correlated inversely with patient age. Neonates had also supranormal percentages of CD45RAhigh, L-selectin+, CD29low/-, CD11alow/-and CD28+ CD8+ T cells whereas the patients (particularly those with chronic GVHD) tended to have subnormal percentages of these CD8+ T cell subpopulations. Contrary to the CD4+ T cells, there was no correlation between the absolute counts of CD45RAhigh, L-selectin+, CD29low/-or CD11alow/-CD8+ T cells and patient age. Whereas the vast majority of neonatal T cells were CD38high, most patient and normal adult T cells were CD38-or CD38intermediate (both CD4+ and CD8+ T cells). We conclude that, in contrast to early life, the production of naive CD4+ T cells is deficient in adult (and particularly elderly) transplant recipients.