A retrospective design was used to evaluate clinical records for a predictive model of lab-confirmed influenza. Children who presented to the ED at Seattle Children's Hospital between January 1, 2001 and May 31, 2005 were eligible for inclusion in the study. The accuracy of four syndrome definitions were compared for identifying lab-confirmed influenza: (1) ILI from chief complaint (CC) or diagnoses codes (''ILI''); (2) ILI from CC alone; (3) febrile illness from CC or diagnoses (''Febrile''); and (4) febrile illness from CC alone. We evaluated syndrome validity over two distinct time periods: (1) the ''discrete'' 2003Á04 influenza season, which had relatively less co-circulation of influenza and respiratory syncytial virus (RSV) compared to most years, and (2) the ''concomitant'' 2000Á05 influenza seasons (excluding 2003Á04), when influenza and RSV co-circulation was high. Analyses during the concomitant years were further stratified by age B5 years and ]5 years. Multiple imputation was used to address missing viral lab results. The imputation model was based on testing guidelines in place at the hospital during the time of study.