OBJECTIVES: In order to estimate the size of the population of infants at risk for seasonal influenza because of ineligibility for vaccination, we developed a method for estimating the cross-sectional number of infants under 6 months of age at any point in time. METHODS: Data on the monthly number of deliveries at gestational ages 23-42 weeks for 2006 (the most recent available) from from publicly accessible birth certificate data from the US National Center for Health Statistics (NCHS) were used.The number of deliveries at each gestational age each calendar week were derived based on the number of days in each month.   Conditional probabilities for survival for each week after birth were derived from the linked birth and death certificate dataset from 2006 for each gestational age.  The number of infants younger than 6 months with a given gestational age at birth at any point in time was calculated using the following formula: Number of infants (gestational age) born (current week) + Number of infants (gestational age) born (current week-1)*1 week survival (gestational age) +…Number of infants (gestational age) born (current week-25)*26 week survival These results were then summed for gestational ages 23-42 weeks to obtain the total number of infants.   Separate calculations were performed for single pregnancies, twins, and higher order multiples. RESULTS: In 2006, the average number of infants 0-6 months of age in any given week was 1.98 million, with seasonal variation reflecting seasonal variability in deliveries. CONCLUSIONS: Estimates of the number of infants within a given age range alive at a given point in time can be derived from routinely collected administrative data.   These estimates, together with estimates of the number of pregnant women, can be used for evaluation of interventions targeting pregnant women and infants.