At the request of the Directorate General for Health, the French Institute for Public Health Surveillance has on several occasions produced estimates of the potential impact of an influenza pandemic. This article presents the methodology used to arrive at these estimates, which were subsequently compared with observed data. The estimates, made prior to the 2009 A(H1N1) pandemic, were based on assumptions that relied primarily on documented pandemics. These assumptions were revised downward as data became available, particularly from affected countries. The observed data are well below the estimates, mainly due to pre-pandemic protection of part of the population against this new virus, as well as a higher proportion of asymptomatic infected individuals than that observed during seasonal influenza. (R.A.)