Bordetella pertussis (Bp) infection in infants and young children can be associated with a significant increase in small lymphocytes with convoluted and cleaved nuclei (SLCCN) in the peripheral blood (PB). Buffy coat smears were studied that were prepared from the PB of 11 children with documented Bp infection, whose ages ranged from one month to four years. The white blood cell count ranged from 8.4 to 72.9 X 10(9)/L, with a mean of 28.6 X 10(9)/L. In all cases, the percentage of PB lymphocytes was in the normal range; the absolute lymphocyte count ranged from 6.5 to 54.8 X 10(9)/L, with a mean of 20.3 X 10(9)/L. SLCCN represented 12-56% of the lymphocyte population. B and T lymphocytes, identified with monoclonal antibodies with the use of an immunoalkaline phosphatase method, accounted for a mean of 21% and 53%, respectively, of the total nucleated cells (TNCs) on buffy coat smears. The T-helper and T-suppressor subsets represented 38% and 16% of the TNCs, respectively, resulting in a CD4-CD8 ratio of 2.4. Most SLCCN were of the T-helper phenotype; SLCCN of the T-suppressor subset and, rarely, of the B-cell type also were identified. These observations document that the lymphocytosis associated with Bp infection in infants and young children is characterized by the presence of morphologically abnormal cells that are predominantly CD4 positive and appear to represent an expansion of an immunophenotypically normal lymphocyte population.