BACKGROUND: Longitudinal information examining the effect of poor infant growth on respiratory syncytial virus (RSV) severity is limited. Children hospitalized with RSV lower respiratory infection represent those at the severe end of the disease spectrum. METHODS: We followed up a cohort of 12,191 infants enrolled in a previous pneumococcal vaccine trial in Bohol, Philippines. Exposure measures were weight for age z-score at the first vaccination visit (median age 1.8 months) as well as the growth (the difference in weight for age z-score) between the first and third vaccination visits. The outcome was hospitalization with RSV lower respiratory infection. RESULTS: Children with a weight for age z-score ≤ -2 at their first vaccination visit had the highest rate of hospitalization with RSV lower respiratory infection, but this association was only evident in children whose mothers had >10 years of education (hazard ratio: 3.38; 95% confidence interval: 1.63-6.98). Children who had lower than median growth between their first and third vaccinations had a higher rate of RSV-associated hospitalization than those with growth above the median (hazard ratio: 1.34; 95% confidence interval: 1.02-1.76). CONCLUSIONS: Poor infant growth increases the risk for severe RSV infection leading to hospitalization. ; Stuart Paynter, Robert S. Ware, Marilla G. Lucero, Veronica Tallo, Hannah Nohynek, Philip Weinstein, Gail Williams, Peter D. Sly and Eric A. F. Simões