In order to understand early events in the immune response to respiratory syncytial virus (RSV) infection, we studied the presence of various chemokines and cytokines in respiratory secretions of human infants with RSV infection. Interferon gamma (IFNgamma) was present in 30/39 (76.9%) subjects tested, but the IFNgamma-inducing cytokines interleukin (IL)12 and IL18 were detectable in 6/40 (15%) and 11/38 (28.9%) subjects, respectively. Quantities of IL12 and IL18 did not correlate with those of IFNgamma. IL18, but neither IFNgamma nor IL12 was found in significantly greater concentrations in subjects with mild, nonhypoxic forms of bronchiolitis than in those with upper respiratory illness alone or hypoxic bronchiolitis. The findings suggest that IFNgamma may be induced independently of the activities of IL12 and IL18 during RSV infection. Immune responses characterized by relatively greater release of IL18 may be associated with milder forms of bronchiolitis.