In 6.5 years of newborn screening for severe combined immunodeficiency in California, 3,252,156 infants had DNA from dried blood spots (DBSs) assayed for T-cell receptor excision circles. Infants with T-cell receptor excision circle values of less than a designated cutoff on a single DBS, 2 DBS samples with insufficient PCR amplification, or known genetic risk of immunodeficiency had peripheral blood complete blood counts and lymphocyte subsets assayed in a single flow cytometry laboratory. Cases in which immune defects were ruled out were available for analysis.We sought to determine reference intervals for lymphocyte subsets in racially/ethnically diverse preterm and term newborns who proved to be unaffected by any T-lymphopenic immune disorder.Effective gestational age (GA) was defined as GA at birth plus postnatal age at the time of sample collection. After determining exclusion criteria, we analyzed demographic and clinical information, complete and differential white blood cell counts, and lymphocyte subsets for 301 infants, with serial measurements for 33 infants. Lymphocyte subset measurements included total T cells, helper and cytotoxic T-cell subsets, naive and memory phenotype of each T-cell subset, B cells, and natural killer cells.Reference intervals were generated for absolute numbers and lymphocyte subsets from infants with effective GAs of 22 to 52 weeks. Sex and ethnicity were not significant determinants of lymphocyte subset counts in this population. Lymphocyte counts increased postnatally.This study provides a baseline for interpreting comprehensive lymphocyte data in preterm and term infants, aiding clinicians to determine which newborns require further evaluations for immunodeficiency.