Data on risk perception and behavioral response in the general population have rarely been collected right from the outset of an epidemic. Instead, they are usually gathered with a substantial delay in the case of influenza A(H1N1) [13] , after the epidemic has run its course, as in the case of SARS [9] , or before sustained human to human transmission is established, as in the case of highly pathogenic avian influenza A(H5N1) [14] . However, the feasibility of halting or mitigating the spread of an infectious disease is highest during the very early phases of an outbreak, and thus data on behavioral response during this time would provide valuable information for public health policy and research. Here, we report the results from an online survey that gathered data (n = 6,249) about risk perception of the outbreak during the first few days of widespread media coverage (April 29 -May 5, 2009) of the emergence of novel swine-origin Influenza A(H1N1).