Hundreds of millions of passengers travel on U.S. airliners annually. These large numbers, together with the close proximity required onboard, raise a concern about microbiologic disease transmission in cabin air. Previous air quality surveys generally concentrated on environmental tobacco smoke and particulate matter. They largely ignored the microorganisms also present. We sampled the microbiologic climate of 45 domestic and international flights. We also sampled common locations in a major southwestern city. The concentration of microorganisms in airline cabin air is much lower than in ordinary city locations. We conclude that the small number of microorganisms found in U.S. airliner cabin environments does not contribute to the risk of disease transmission among passengers.