We describe a 17-day-old newborn with fever and peripheral blood lymphocytosis. The circulating lymphocytes were large with lobulated and nucleolated nuclei. Their immature and uniform appearance raised the possibility of malignant lymphoma in the leukemic phase. Immunophenotypic study, however, showed that the lymphocytes were CD19(+) B cells with expression of both kappa and lambda light chains. Molecular biology study confirmed a polyclonal nature of the immunoglobulin heavy-chain gene. Cytogenetic analysis showed a normal karyotype, and viral cultures and serologic studies yielded negative results. The polyclonal lymphocytosis was self-limiting and disappeared within a month.