On April 12, 2009, Mexico responded to a request for verification by the World Health Organization (WHO) of an outbreak of acute respiratory illness in the small community of La Gloria, Veracruz. During April 15-17, the Mexico Ministry of Health received informal notification of clusters of rapidly progressive severe pneumonia occurring mostly in Distrito Federal (metropolitan Mexico City) and San Luis Potosi. In response, on April 17, Mexico intensified national surveillance for acute respiratory illness and pneumonia. During April 22-24, novel influenza A (H1N1) virus infection, previously identified in two children in the United States, was confirmed in several patients. This report updates a previous report on the outbreak in Mexico and summarizes public health actions taken to date by Mexico to monitor and control the outbreak. During March 1-May 29, national surveillance identified 41,998 persons with acute respiratory illness; specimens from 25,127 (59.8%) patients were tested, of which 5,337 (21.2%) were positive for novel influenza A (H1N1) virus infection by real-time reverse transcription--polymerase chain reaction (rRT-PCR). As of May 29, 97 patients with laboratory-confirmed infection had died. Epidemiologic evidence to date suggests that the outbreak likely peaked nationally in late April, although localized cases continue to be identified.