BACKGROUND: Severe bronchiolitis (ie, bronchiolitis requiring hospital admission) is thought to markedly increase asthma risk, with 30%-50% developing asthma by age 5 years. To date, studies of this association are small, and most are from outside the United States. OBJECTIVE: The objective of this study was to investigate the association between severe bronchiolitis and risk of asthma in a US birth cohort. METHODS: We studied a cohort nested within the Massachusetts General Hospital Obstetric Maternal Study (MOMS), a prospective cohort of pregnant women enrolled during 1998-2006. Children of mothers enrolled in MOMS were included in the analysis if they received care within our health system (n = 3653). Diagnoses and medications were extracted from the children's electronic health records; we also examined pregnancy and perinatal risk factors collected for the underlying pregnancy study. RESULTS: The birth cohort was 52% male, 49% white, and 105 infants (2.9%) had severe bronchiolitis. Overall, 421 children (11.5%) developed asthma by age 5 years. Among the children with severe bronchiolitis, 27.6% developed asthma by age 5 years. In multivariable logistic regression adjusting for 12 risk factors, severe bronchiolitis remained a strong risk factor for developing asthma by age 5 years (odds ratio 2.57; 95% confidence interval 1.61-4.09). Conclusions: In a large Boston birth cohort, the frequency of severe bronchiolitis and childhood asthma was similar to published data. Among children with severe bronchiolitis, the risk of developing asthma was lower than prior studies but still high (27.6%). This difference may be due to different study designs, populations, and outcome definitions studied. (C) 2016 American Academy of Allergy, Asthma & Immunology.