This report provides an update to the international situation as of September 18, 2009. As of September 13, 2009, the World Health Organization (WHO) regions have reported more than 296,471 laboratory-confirmed cases of 2009 H1N1 influenza virus (2009 H1N1) with at least 3,486 deaths, which is an increase of at least 18,864 cases and 281 deaths since September 6th. The laboratory-confirmed cases represent a substantial underestimation of total cases in the world, as many countries focus surveillance and laboratory testing only on people with severe illness. The 2009 H1N1 influenza virus continues to be the dominant influenza virus in circulation in the world. Since April 2009, 60.7% of influenza specimens reported to WHO were 2009 H1N1 viruses. In temperate regions of the Southern Hemisphere, disease due to 2009 H1N1 is largely declining. In tropical regions, there is still substantial disease due to 2009 H1N1. In temperate regions of the Northern Hemisphere, there is some increased disease activity due to 2009 H1N1, including in parts of the United States, Canada and Europe.