This study sought to estimate influenza vaccination target population sizes in Spain for the 2006-2007 season, based firstly on current vaccine recommendations, and secondly, on the hypothetical assumption that Advisory Committee on Immunization Practices (ACIP) recommendations were to be implemented. We estimate that under present Spanish guidelines, 41% of the population should be vaccinated against influenza in the 2006-2007 season. Of those eligible for vaccination, 41% are aged >or=65 years, 16% are aged <65 years and suffer from a chronic condition, and 36% are healthy household contacts aged <65 years. If the ACIP recommendations were implemented in Spain, the target population size would increase to 26,761,506 persons (61%), and of those eligible for vaccination, 57% would fall within the age-based recommendations (ages 6-59 months or >or=50 years), 29% would come under the healthy household contacts category, and only 5% would qualify due to suffering from a medical condition. We thus conclude that approximately 18 million persons, 41% of the Spanish population, should receive influenza vaccination in the 2006-2007 season under present Spanish guidelines. With the estimated number of doses to be distributed during this season (10 million), compliance with current recommendations would amount to only 56% at best. Extending universal vaccination to the 50-64 age group should be considered as an option for improving influenza vaccination coverage, particularly among high-risk patients, because a greater proportion of persons would receive the recommended dose by becoming eligible for the more effective age-based strategies.