A total of 442 infl uenza-associated deaths among children (<18 years old) and young adults (18-40 years old) residing in the United States were reported to CDC for the 1998-99 through 2007-08 infl uenza seasons; of these, 105 cases with laboratory-confi rmed infl uenza infection had suffi cient tissue specimens available for DNA extraction and constitute the analytic dataset for this study. Fatal infl uenza cases were considered laboratory confi rmed if a positive test result for infl uenza by viral culture, immunohistochemical analysis, or reverse transcription PCR (RT-PCR) had been documented. These represented 1) fatal pediatric cases reported to CDC during the 2003-04 infl uenza season when CDC conducted surveillance for infl uenza-associated pediatric deaths as part of an emergency response effort; 2) fatal pediatric cases identifi ed through national surveillance since 2004 when pediatric infl uenza-associated death was made nationally notifi able in the United States; or 3) fatal cases of infl uenza among young adults at any point in time or among children before 2003 whose case reports and specimens were received by the CDC Infectious Diseases Pathology Branch on a case-by-case basis.