VFDB is a comprehensive database for bacterial virulence factors (VFs), providing a centralized resource for scientists to access current knowledge about VFs from various bacterial pathogens. The database is user-friendly, allowing searches by browsing each genus or typing keywords, and includes a BLAST search tool against all known VF-related genes. VFDB offers a unified gateway to store, search, retrieve, and update information about VFs from various bacterial pathogens. The database includes information on 16 important bacterial pathogens, virulence-associated genes, protein structural features, functions, mechanisms, and important literature. Pathogenicity islands (PAIs), which are clusters of genes encoding virulence traits, are also included. The database is fully searchable by query text or by function categories of VFs, as well as by BLAST comparison against all VF-related genes. VFDB is built on a Linux operating system with MySQL as the relational database, and Perl is used for communication with the database, dynamically generating circular or linear maps and sending pages to the Apache web server. VFDB can be freely accessed through a web browser at http://www.mgc.ac.cn/VFs/. Users can browse and search the database. The database provides a powerful search engine for users to extract information from the database quickly through text search, BLAST search, and VFs function category search. VFDB is a specialized database aimed at making a comprehensive repository of known VFs, complemented by data extraction and analysis tools. The current statistics of VFDB are summarized in Table 1, and the most up-to-date status of the database is available at http://www.mgc.ac.cn/cgi-bin/VFs/status.cgi. Researchers are invited to deposit their new results of VFs at VFDB. Current VFDB contains information of VFs from 16 important bacteria, while more pathogens, such as Chlamydia and Mycoplasm, are under way to be included into the database in the next release. As regulation of VFs expression is another important part of the virulence mechanisms, we plan to include more detailed information in this area in the future. This project is supported by the National Basic Research Priorities Program and the High Technology Research and Development Program from the Ministry of Science and Technology of China.VFDB is a comprehensive database for bacterial virulence factors (VFs), providing a centralized resource for scientists to access current knowledge about VFs from various bacterial pathogens. The database is user-friendly, allowing searches by browsing each genus or typing keywords, and includes a BLAST search tool against all known VF-related genes. VFDB offers a unified gateway to store, search, retrieve, and update information about VFs from various bacterial pathogens. The database includes information on 16 important bacterial pathogens, virulence-associated genes, protein structural features, functions, mechanisms, and important literature. Pathogenicity islands (PAIs), which are clusters of genes encoding virulence traits, are also included. The database is fully searchable by query text or by function categories of VFs, as well as by BLAST comparison against all VF-related genes. VFDB is built on a Linux operating system with MySQL as the relational database, and Perl is used for communication with the database, dynamically generating circular or linear maps and sending pages to the Apache web server. VFDB can be freely accessed through a web browser at http://www.mgc.ac.cn/VFs/. Users can browse and search the database. The database provides a powerful search engine for users to extract information from the database quickly through text search, BLAST search, and VFs function category search. VFDB is a specialized database aimed at making a comprehensive repository of known VFs, complemented by data extraction and analysis tools. The current statistics of VFDB are summarized in Table 1, and the most up-to-date status of the database is available at http://www.mgc.ac.cn/cgi-bin/VFs/status.cgi. Researchers are invited to deposit their new results of VFs at VFDB. Current VFDB contains information of VFs from 16 important bacteria, while more pathogens, such as Chlamydia and Mycoplasm, are under way to be included into the database in the next release. As regulation of VFs expression is another important part of the virulence mechanisms, we plan to include more detailed information in this area in the future. This project is supported by the National Basic Research Priorities Program and the High Technology Research and Development Program from the Ministry of Science and Technology of China.