Respiratory-tract infections remain the foremost cause of death in young children and are a major obstacle to achievement of Millennium Development Goal 4—to reduce under-5 mortality by two-thirds by 2015. 1–3 Respiratory syncytial virus (RSV) is the leading cause of acute lower respiratory-tract infections and admissions to hospital worldwide. It is the only agent of the three major organisms that causes death from respiratorytract infections—RSV, Streptococcus pneumoniae, and Haemophilus influenzae—for which no vaccine is available. 2, 3 RSV has been prioritised for control and vaccine develop ment by global and national health organisations for more than 20 years. 4, 5 Successful immunisation against RSV was predicted, in 1994, to be achieved within 10 years. 6 The current burden of RSV in young children indicates the potential of an effective vaccine. However, information from developing countries—where most of the world’s children live—is scarce, making estimation of childhood disease and mortality a challenge. 7 In The Lancet today, Harish Nair and colleagues8 undertake this challenge. They aimed to quantify the burden of RSV in 2005 by systematic review of articles published from 1995 to 2009. They examined rates of RSV-associated acute lower respiratory-tract infections, serious (leading to admission) infections of this type, and deaths in children younger than 5 years. The article is singular by its use of several analytical approaches and by inclusion of unpublished data from an additional ten population-based studies. Data for industrialised and developing countries were compared and, importantly, were examined by global region. In today’s study, RSV was estimated in 2005 to cause 33· 8 million cases of acute lower respiratorytract infection in children younger than 5 years, accounting for 22% of all infections of this type and 3–9% of deaths. In previous studies, the major contribution of RSV to childhood viral acute lower respiratory-tract infections has been revealed. 9–11 But Nair and colleagues’ data indicate that the global burden of RSV is rising, and this increase is disproportionate in resource-poor nations. 96% of acute lower respiratory-tract infections caused by RSV and 99% of fatal cases were in developing countries. This finding is only partly explained by recognition that 90% of children younger than 5 years currently reside in these areas.