Since the emergence of the new influenza A(H1N1)v virus was first recognized, initial quantitative data on its transmissibility have become available. This article provides a summary of the available knowledge as well as guidance on interpreting the published figures. Regarding the potential for the infection to spread, initial estimates indicate a reproduction number ranging from 1.4 to 3.1, higher than that of seasonal influenza and close to that of past pandemics; knowledge regarding the generation interval (1.9 days in Mexico, 3.5 days in Spain) remains limited. These preliminary estimates vary depending on populations with differing social contact structures and frequencies; they will therefore continue to evolve as the virus spreads. Estimates of the health impact are also currently imprecise. However, available data suggest higher transmissibility and/or susceptibility among children and adolescents. Initial estimates of case fatality in Mexico suggest a rate higher than that of seasonal influenza, but this phenomenon has not been observed elsewhere. Monitoring these parameters over time is essential to detect any changes that could guide and adapt pandemic management and control measures. (R.A.)