The risk of transmission of severe acute respiratory syndrome (SARS) on airplanes is of major concern to the public and airline industry. We examined data from flights to Singapore with SARS patients on board in order to assess this risk. In-flight transmission occurred only in one of the three flights with symptomatic SARS patients on board. The incidence was estimated to be 1 out of 156 passengers. The risk of in-flight transmission of SARS appears to be far lower than that reported for influenza, but may be increased with superspreaders on board.