We applied pulsed-field gel electrophoresis (PFGE) to the investigation of diffuse outbreaks of illness due to Shiga toxin?producing Escherichia coli O157:H7 (STEC O157) in Japan and used these data to develop a database of STEC O157 PFGE patterns and associated clinical and microbiologic information to facilitate the recognition of geographic and temporal clusters of cases based on their PFGE profiles. This project has evolved into a subtyping network called PulseNet Japan that is cooperatively run by National Institute of Infectious Diseases (NIID) and the local Health Institutes and the Ministry of Health, Labor and Welfare. Although our domestic PFGE network that utilized locally developed PFGE protocols was effective in recognizing diffuse outbreaks of STEC O157 within Japan, we decided to adopt the standardized PFGE protocols from PulseNet USA and collaborate closely with the Centers for Disease Control and Prevention (CDC) in the United States to facilitate recognition of international clusters of STEC O157 and their investigations.