• A historical cohort study was conducted to measure lower respiratory tract illness (LRTI) up to nine years following mild bronchiolitis and to discover attributes that predict an increased risk for LRTI in childhood. The hypothesis assessed was that the occurrence of bronchiolitis predicts LRTI following the second birthday (childhood LRTI) after adjusting for potentially confounding variables such as a family history of respiratory allergy. Fifty-three children (index subjects) who were seen for bronchiolitis at a suburban community pediatric practice were compared on the basis of childhood LRTI with a control group of 159 children. Chart review, when children were a mean of 8 years old, determined the frequency of childhood LRTI. The mean frequency of childhood LRTI was greater in index subjects than in control subjects (1.62 vs 0.98). This difference remained after adjusting for potentially confounding variables. The yearly occurrence of any LRTI was significantly more common in index subjects through the third year of life (38% vs 15%). A tendency for a more common occurrence of any LRTI was noted through the fifth year (25% vs 16%), but not thereafter. Further analysis of index subjects disclosed that only those who experienced a subsequent LRTI before the second birthday were at an increased risk for the development of childhood LRTI.