OBJECTIVES: Lollapalooza (LP) is an annual 3-day outdoor music festival in Chicago. Underage drinking and drug use are believed to be common, but the burden on emergency departments (EDs) has not been documented. We assessed the burden on health care resources associated with this music festival. METHODS: We performed a retrospective chart review of adolescent (aged 11-20 years) ED visits during LP weekend and 4 summer comparison weekends at Lurie Children's Hospital in Chicago, Ill, in 2014 (n = 356). We then analyzed adolescent alcohol- and drug-related hospital visits to all Chicago hospitals for each weekend in 2014 0 using Illinois hospital discharge data. RESULTS: Adolescents accounted for a greater proportion of our ED visits during LP weekend than comparison weekends (25% vs 19%, P < 0.02). Lollapalooza weekend patients were more likely female (P = 0.025), older (P = 0.0067), more often unsupervised (P < 0.0001), and less likely to live in the city (P < 0.001) than adolescents seen during comparison weekends. Thirty-one underage adolescents who attended LP were treated in our ED; 84% were intoxicated (blood alcohol content, 88-328 mg/dL). Citywide there was an 11-fold increase in adolescent alcohol-related hospital visits during LP weekend compared with an average weekend. Drug intoxication was much less common. CONCLUSIONS: Adolescents seen in our ED the weekend of LP were older, more often female, frequently unsupervised, and less likely to be city residents than those seen during comparison weekends. Those who attended LP had high rates of alcohol intoxication. This surge of intoxicated adolescent patients affected numerous EDs in the city.