Most data concerning immunopathogenetic mechanisms involved in respiratory syncytial virus (RSV) infection are derived from animal studies. In infants with RSV bronchiolitis the target organ i.e. the airway is hard to explore. We looked for specific alterations in peripheral blood lymphocyte subpopulations in infants hospitalized for RSV bronchiolitis. Flow cytometric analysis with a large panel of monoclonals was performed on peripheral blood lymphocytes in thirty-two infants (mean age: 4.9 months) admitted for RSV bronchiolitis. Data collected on admission were compared with age-matched control values and also with results obtained at the end of the first week of hospitalization. Differences between age-groups (older or younger than 4 months) and between clinical subgroups (clinical severity score more or less than 6) were looked for. In the group of infants as a whole, regardless of age and clinical score the number of CD4+ cells on admission was significantly elevated compared to normal values for age (p<.0001) including a high fraction of the naive suppressor-inducer subpopulation (CD4+/CD45RA+) and a low fraction of the reciprocal memory helper-inducer subpopulation (CD4+/CD29+). Within the CD8+ cell population the number of T cells with cytotoxic activity (CD8+/S6F1+) was significantly elevated (p<.0001) as were other types of cytotoxic cells. A significant decrease (p<.0001) in the proportion of the precursor/suppressor-effector subpopulation (CD8+/S6F1-) was seen. Absolute numbers and percentages of CD19+ B cells were significantly elevated (p<.0001) with a significant increase in the CD5+ subfraction (p<.0001) as well as in the CD10+ subfraction (p<.0001). In the older age group immunophenotypic cytotoxicity was more pronounced with increased clinical score. During recovery the CD45RA+:CD29+ ratio tended to normalize within the CD4+ T cells. Within the B lymphocyte subsets significant increase in the CD19+/CD5+ fraction (p<.05) was seen. We conclude that there are significant changes in the number of peripheral blood lymphocyte subsets in infants with RSV bronchiolitis as compared to age-related controls. We hope that present data could be useful in further exploration of RSV immunology in humans. A possible link between RSV bronchiolitis and the subsequent development of atopy is mentioned.