Although community violence may influence asthma morbidity by increasing stress, no study has assessed exposure to gun violence and childhood asthma. We examined whether exposure to gun violence is associated with asthma in children, particularly in those reporting fear of leaving their home. Case-control study of 466 children aged 9-14 years with (n=234) and without (n=232) asthma in San Juan, Puerto Rico. Lifetime exposure to gun violence was defined as hearing a gunshot more than once. We also assessed whether the child was afraid to leave his/her home because of violence. Asthma was defined as physician-diagnosed asthma and wheeze in the prior year. We used logistic regression for the statistical analysis. All multivariate models were adjusted for age, gender, household income, parental asthma, environmental tobacco smoke, prematurity and residential distance from a major road. Cases were more likely to have heard a gunshot more than once than control subjects (n=156 or 67.2% vs. n=122 or 52.1%, P<0.01). In a multivariate analysis, hearing a gunshot more than once was associated with asthma (odds ratio [OR]=1.8, 95% confidence interval [CI]=1.1-1.7, P=0.01). Compared with children who had heard a gunshot not more than once and were not afraid to leave their home because of violence, those who had heard a gunshot more than once and were afraid to leave their home due to violence had 3.2 times greater odds of asthma (95% CI for OR=2.2-4.4, P<0.01). Exposure to gun violence is associated with asthma in Puerto Rican children, particularly in those afraid to leave their home. Stress from such violence may contribute to the high burden of asthma in Puerto Ricans.