ARI/pneumonia has been the leading cause of morbidity and mortality in under five children for a long time. ARI control/Integrated Management of Childhood Illness (IMCI) program have been in progress in the country for more than a decade. There have been recent reports of outbreak of RSV bronchiolitis in the country. This study was done to determine the causes of death and morbidity of under five children with possible explanations. A cross sectional national survey was conducted between January to December 2003. Twelve out of 64 districts were randomly selected. Survey was conducted in 171,366 households having 820,347 populations covering 90,357 under five children including 16,193 infants. Data were collected with three sets of forms: screening form, verbal diagnosis form and verbal autopsy form. Consensus was achieved on the diagnosis after analyzing the forms by a group of pediatricians. We also analyzed various reports on childhood morbidity and mortality of Director General of Health Services (DGHS), Bangladesh. ARI/pneumonia was found to be leading cause of under five morbidity and mortality. Most of the pneumonia deaths occurred under two years of age and it accounted for 77.5% of deaths in the first year of life. Again, children of 2-6 months of age were found to have 2.6 times higher chance of death due to pneumonia. The peak age of bronchiolitis is 2-6 months. The proportions of pneumonia deaths of children under two years were mostly in winter and spring (64.3%), the peak season of RSV bronchiolitis. The cases of bronchiolitis were misclassified as pneumonia and deaths from bronchiolitis have merged with pneumonia deaths giving rise to the bulk of ARI/pneumonia deaths. The disease entity of bronchiolitis deserves its due importance as regards diagnosis, documentation, management and counseling.