Throughout March and April 2009, international air travelers departing from Mexico were unknowingly transporting a novel influenza A (H1N1) virus1 to cities around the world. We analyzed the flight itineraries for all passengers departing from commercial airports in Mexico between March and April 2008, using data from the International Air Transport Association (IATA). The purpose of this analysis was to show how travelers — and consequently to predict how H1N1 — would disseminate worldwide during the initial wavefront of this epidemic. We analyzed IATA data from March and April 2008 for the following reasons: the data accounted for more than 95% of all passenger trips worldwide via commercial airlines, they included information on the flight origins and destinations of actual passengers, data on passenger itineraries for the period from March through April 2009 were unavailable at the time of this analysis, the global pattern of passenger departures from Mexico between March and April varies minimally from year to year, and the epidemic in Mexico was largely unrecognized during the period from March through April 2009, with passenger departures presumably following their usual seasonal pattern.