CD31+CD45RA+RO– lymphocytes contain high numbers of T cell receptor circle (TREC)-bearing T cells; however, the correlation between CD31+CD4+ lymphocytes and TREC during aging and under lymphopenic conditions has not yet been sufficiently investigated. We analyzed TREC, telomere length and telomerase activity within sorted CD31+ and CD31– CD4+ lymphocytes in healthy individuals from birth to old age. Sorted CD31+CD45RA+RO– naive CD4+ lymphocytes contained high TREC numbers, whereas CD31+CD45RA–RO+ cells (comprising ⩽5% of CD4+ cells during aging) did not contain TREC. CD31+ overall CD4+ cells remained TREC rich despite an age-related tenfold reduction from neonatal (100 : 1000) to old age (10 : 1000). Besides a high TREC content, CD31+CD45RA+RO–CD4+ cells exhibited significantly longer telomeres and higher telomerase activity than CD31–CD45RA+RO–CD4+ cells, suggesting that CD31+CD45RA+RO–CD4+ cells represent a distinct population of naive T cells with particularly low replicative history. To analyze the value of CD31 in lymphopenic conditions, we investigated six children after allogeneic hematopoietic stem cell transplantation (HSCT). Reemerging overall CD4+ as well as naive CD45RA+RO–CD4+ cells predominantly expressed CD31 and correlated well with the recurrence of TREC 5–12 months after HSCT. Irrespective of limitations in the elderly, CD31 is an appropriate marker to monitor TREC-rich lymphocytes essentially in lymphopenic children after HSCT.