T cell receptor excision circles (TRECs) are small circularized DNA elements produced during rearrangement of T cell receptor (TCR) genes. Because TRECS are fairly stable, do not replicate during mitosis, and are not diluted during division of naïve T cells1, they are suitable for assessing the number of newly formed T cells 2. In this study, we detected TRECs in 475 healthy Chinese children aged 0–18 years in different clinical settings. We found a strong correlation between TRECs levels and peripheral CD4 naïve T cell numbers, but not between TRECs levels and effector or memory CD4 and CD8 T cell numbers. TRECs levels fell significantly compared with normal controls in patients with severe combined immunodeficiencies (SCID) (n=7), wiskott-aldrich syndrome (WAS) (n=22), or activated PI3Kδ syndrome (APDS) (n=5). TRECs levels in those with signal transducer and activator of transcription 1 (STAT1) deficiency (n=8) decreased or did not change significantly, a finding consistent with that for CD4 naïve T cells. We also measured TRECs levels in seven PIDs after hematopoietic stem cell transplantation (HSCT) (WAS=5; chronic granulomatous disease (CGD)=2), and found the complications after HSCT may reduce TRECs levels by interfering with production of naïve T cells. In conclusion, we established reference values for TRECs, which can be used to screen for primary immunodeficiency diseases (PIDs) during early life and track immune reconstitution after HSCT.