It is a common and well-known fact that infants and preschool children undergo frequent episodes of upper respiratory tract infections. The majority of these children do not have a recognized immunodeficiency. The aim of the present study was to evaluate the effects of frequent upper respiratory tract infections on cellular immunity, using peripheral blood lymphocyte subsets and activation markers as defining parameters. The study group consisted of 16 children (aged 2-6 years) with frequent upper respiratory tract infections; 30 age-matched healthy children served as controls. Peripheral blood T, B, NK cells; T lymphocyte subsets; naive and memory cells; and activation markers were analyzed by using monoclonal antibodies and flow cytometry. White blood cell count (WBC) was found to be markedly increased in the study group compared to controls (p < 0.05). The absolute number of lymphocytes was also higher than that of the healthy children. The relative size of the CD3+CD8+ T lymphocytes and the relative and absolute numbers of CD3-CD16+56+ NK cells were found to be higher in patients than the controls. All the remaining percentages and numbers of the T cell subgroups including naive and memory cells and B lymphocytes did not show any difference, while CD3+CD25+ cell numbers were markedly increased (p < 0.05). In conclusion, the examination of peripheral blood lymphocyte subsets in children with frequent upper respiratory tract infections is important in evaluating cellular immune alterations due to antigenic stimulation; however, it is neither essential nor cost-effective in the management of the disease. This study has shown that both the percentage and absolute numbers of peripheral blood lymphocyte subsets maintain their normal status in children with frequent upper respiratory tract infections.