Objective: To determine the impact of influenza coinfection on outcomes for children with complicated pneumonia. Design: Retrospective cohort study. Setting: Forty children's hospitals that contribute data to the Pediatric Health Information System. Participants: Children discharged from participating hospitals between January 1, 2004, and June 30, 2009, with complicated pneumonia requiring a pleural drainage procedure. Main Exposure: Influenza coinfection. Main Outcome Measures: Intensive care unit admission, receipt of mechanical ventilation, receipt of vasoactive infusions, receipt of blood product transfusions, in-hospital death, readmission within 14 days of hospital discharge, hospital length of stay, and cost of hospitalization. Results: Overall, 3382 of 9680 children with complicated pneumonia underwent pleural fluid drainage; 105 patients (3.1%) undergoing pleural drainage had influenza coinfection. A bacterial pathogen was identified in 1201 cases (35.5%); the most commonly identified bacteria were Staphylococcus aureus in children with influenza coinfection (22.9% of cases) and Streptococcus pneumoniae in children without coinfection (20.0% of cases). In multivariable analysis, influenza coinfection was associated with higher odds of intensive care unit admission and receipt of mechanical ventilation, vasoactive infusions, and blood product transfusions as well as higher costs and a longer hospital stay. Children with influenza coinfection were less likely to require readmission, although there was a trend toward higher odds of mortality for patients with coinfection. In a subanalysis stratified by bacteria, outcomes remained worse for coinfected children in the subgroups of children with S aureus and with no specified bacteria. Conclusions: Influenza coinfection occurred in 3.1% of children with complicated pneumonia. Clinical outcomes for children with complicated pneumonia and influenza coinfection were more severe than for children without documented influenza coinfection.