• Bronchiolitis is a common lower respiratory tract illness in infants and has substantial acute morbidity and sequelae. To identify risk factors for bronchiolitis, a case-control study was conducted in which 53 subjects who had bronchiolitis were matched with two controls who had no bronchiolitis in infancy. In multivariate analysis, direct effects of passive smoking and older siblings achieved statistical significance. Family history of asthma appeared to interact with older siblings. Among subjects without a family history of asthma, statistically significant predictors proved to be older siblings (odds ratio, 2.31) and passive smoking (odds ratio, 3.87). Among subjects with a family history of asthma, older siblings proved to be an even stronger predictor (odds ratio, 46.81), while the odds ratio for passive smoking did not change much (odds ratio, 4.03). The combined presence of older siblings and passive smoking yielded an odds ratio of 8.94 among subjects without a family history of asthma and 181.67 among subjects with a family history of asthma. Analysis provided risk estimates that were particularly high for certain groups. Among infants with a family history of asthma, 49% who have an older sibling may develop bronchiolitis. If they are also exposed to cigarette smoke, almost 80% may develop bronchiolitis. Among infants without a family history of asthma, bronchiolitis may develop in 46% of infants if there is both an older sibling and exposure to smoke. Exposure of infants to cigarette smoke might diminish more rapidly if clinicians and parents were aware of such high risks. Efforts to reduce morbidity from bronchiolitis in infants might best be directed at the reduction of smoking in families with previous children, particularly if there is a family history of asthma, and at methods that protect infants from respiratory virus carried by siblings.