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Objective: To estimate the incidence of influenza-virus-associated severe pneumonia among Salvadorian children aged < 5 years., Methods: Data on children aged < 5 years admitted with severe pneumonia to a sentinel hospital in the western region were collected weekly. Nasal and oropharyngeal swab specimens were collected from a convenience sample of case patients for respiratory virus testing. A health-care utilization survey was conducted in the hospital catchment area to determine the proportion of residents who sought care at the hospital. The incidence of influenza-virus-associated severe pneumonia among all Salvadorian children aged < 5 years was estimated from surveillance and census data, with adjustment for health-care utilization. Influenza virus strains were characterized by the United States Centers for Disease Control and Prevention to determine their correspondence with northern and southern hemisphere influenza vaccine formulations., Findings: Physicians identified 2554 cases of severe pneumonia. Samples from 608 cases were tested for respiratory viruses and 37 (6%) were positive for influenza virus. The estimated incidence of influenza-virus-associated severe pneumonia was 3.2 cases per 1000 person-years (95% confidence interval, CI: 2.8-3.7) overall, 1.5 cases per 1000 person-years (95% CI: 1.0-2.0) during 2008, 7.6 cases per 1000 person-years (95% CI: 6.5-8.9) during 2009 and 0.6 cases per 1000 person-years (95% CI: 0.3-1.0) during 2010. Northern and southern hemisphere vaccine formulations matched influenza virus strains isolated during 2008 and 2010., Conclusion: Influenza-virus-associated severe pneumonia occurred frequently among young Salvadorian children during 2008-2010. Antigens in northern and southern hemisphere influenza vaccine formulations corresponded to circulating strains.