Samples of nasopharyngeal secretions from a group of 73 infants with bronchiolitis or upper respiratory illness alone during infection with respiratory syncytial virus were analyzed for leukotriene C4 (LTC4) content using a reverse-phase high-pressure liquid chromatography assay with confirmation by radioimmunoassay. Titers of respiratory syncytial virus (RSV)-specific IgE in nasopharyngeal secretion (NPS) specimens were determined using an enzyme-linked immunosorbent assay. The highest concentrations of LTC4 were found in the first 3 to 8 days after the onset of illness, and LTC4 was detectable in progressively lower concentrations in samples obtained up to 28 days after the onset of illness. LTC4 was detected in samples of NPS obtained in the acute phase of illness from 67% of infants with bronchiolitis due to RSV and in 33% of samples of NPS obtained during the same interval from infants with upper respiratory illness alone (p less than 0.025). Concentrations of LTC4 in children with bronchiolitis were 5-fold higher (1271 pg/ml) than the mean concentration of LTC4 in children with upper respiratory illness (224 pg/ml, p less than 0.02). LTC4 was detected in 83% of the children developing an RSV-IgE response and in 24% of subjects not developing an RSV-IgE response (p less than 0.001). Quantities of LTC4 measured in NPS were directly correlated with the magnitude of the RSV-IgE response in secretions (r = 0.33, p less than 0.02). These studies lend support to previous investigations suggesting that severe bronchiolitis due to RSV results from IgE-mediated hypersensitivity reactions to viral antigens, with release of chemical mediators of airway obstruction.(ABSTRACT TRUNCATED AT 250 WORDS)