We included patients with a diagnosis of influenza (ICD9 codes 487.0, 487.1, 487.8, 488.0 or a textual variant of 'influenza', excluding 'H. influenza'). We also focused on 2008 data exclusively, since at the time of the analysis, more recent CHARS data were unavailable. Of the 20 hospitals in King County, 10 provided admissions data in 2008, but data from only 9 hospitals were available in CHARS for comparison. For each of the 180 influenza hospitalizations identified by the PHSKC system, we manually attempted to find a matching record in CHARS according to hospital name, discharge month/ year, age, sex and zip code. We flagged all influenza admissions in the PHSKC system that did not have a matching record in CHARS. Next, we asked hospitals with unmatched records to reverse-identify patients and retrieve their medical charts for PHSKC review.