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Pneumonia is the leading cause of child mortality worldwide. The role of influenza in childhood pneumonia in tropical developing countries is poorly understood. We undertook population-based surveillance among low-income urban preschool children to determine its role in childhood pneumonia.Longitudinal prospective active surveillance was conducted among randomly selected households in a poor urban area of Dhaka. Nasopharyngeal washes were collected from 1 in 5 children for influenza culture isolation. Clinical data were collected at clinical presentation and through the illness course.From April 1, 2004 through December 31, 2007, 12,062 children presented in clinic with eligible febrile and respiratory illnesses, from whom 321 influenza isolates were obtained from 2370 nasopharyngeal washes (13.5%), representing 16,043 child-years of observation (adjusted influenza incidence 102 episodes/1000 child-years). There were 8198 pneumonia episodes during the period (pneumonia incidence 511 episodes/1000 child-years). Ninety influenza-positive children (28%) developed pneumonia during their illness. Among influenza culture-positive children, those with pneumonia were younger than those without (23.4 vs. 29.7 months, ANOVA: P < 0.001). Pneumonia was more commonly associated with Influenza A (H3N2) than either A (H1N1) or B infections (age-adjusted relative odds (RO) 2.98, [95% CI: 1.56, 5.71] and 2.75, [95% CI: 1.52, 4.98], respectively). Influenza was associated with 10% all childhood pneumonia.Influenza is a major contributor to childhood pneumonia both through high influenza infection incidence and high pneumonia prevalence among infected children. Its contribution to early childhood pneumonia appears under-appreciated in high pneumonia-endemic tropical settings. Influenza vaccine trials against childhood pneumonia are warranted.