The detailed immunophenotype of peripheral blood lymphocytes from a neonate with pertussis was determined by flow cytometry and compared with results from cord blood from healthy newborns. Most (72%) of the lymphocytes were CD3+ T cells with a normal CD4/CD8 ratio (2.5). The T cells were largely HLA-DR negative and CD45RA+, consistent with unstimulated naïve T cells. Almost all of the CD4+ T cells were Leu8 (L-selectin, CD62L) negative, while almost all of the CD8+ T cells were CD28+. There was no increase in CD7- CD4+ T cells (Th2-like). No relative increase in CD16/56+ NK cells (5%) or CD19/20+ B cells was seen. The most dramatic finding in this case was the remarkable lack of expression of L-selectin by the T cells. L-selectin expression is associated with homing of peripheral blood lymphocytes to lymph nodes. The dramatic reduction in L-selectin expression of the T lymphocytes in pertussis, perhaps induced by pertussis toxin, likely prevents homing of the T cells to peripheral lymphoid tissues and provides a likely explanation for the marked lymphocytosis noted in this disease.