Acute viral respiratory illness during infancy has been implicated as a precursor for subsequent lower respiratory morbidity in childhood. A prospective, longitudinal study of respiratory function, airway responsiveness, and lower respiratory illness during early childhood was performed in a cohort of 253 healthy infants to characterise those who experienced bronchiolitis. Seventeen infants (7% of the cohort), were given a diagnosis of bronchiolitis during the first two years of life with two (1%) requiring hospital admission. Seventy one per cent of those infants with bronchiolitis had a family history of atopy, 53% of asthma, and 29% had a mother who smoked cigarettes. These family history characteristics in this group with bronchiolitis were not different from the rest of the cohort. There were also no differences in the number of older siblings, the number breast fed, the duration of breast feeding, or socioeconomic status of the families between those that did and did not get bronchiolitis. Respiratory function was assessed at 1, 6, and 12 months of age. Maximum flow at functional residual capacity (VmaxFRC) was measured using the rapid thoracic compression technique. Resistance (Rrs) and size corrected compliance (Crs/kg) were obtained from a single brief occlusion at end inspiration. Airway responsiveness was assessed by histamine inhalation challenge and the provocation concentration of histamine resulting in a 40% fall on VmaxFRC from baseline (PC40) was determined. Respiratory measurements were ranked into terciles to assess the distribution of infants who developed bronchiolitis through the cohort. Cough and wheeze were noted to be frequent before the episode of bronchiolitis. This study has demonstrated that infants who develop bronchiolitis have evidence of pre-existing reduced respiratory function and lower respiratory symptoms. It is proposed that bronchiolitis, although potentially contributory, is not usually causative of subsequent lower respiratory morbidity.