Atypical hemolytic uremic syndrome (aHUS) mutations have been reported in the complement regulatory proteins factor H, factor I, and membrane cofactor protein (MCP). Mutations within factor H are also associated with membranoproliferative glomerulonephritis and age-related macular degeneration. The increasing amount of information on aHUS requires organization if it is to be usable. Accordingly, an interactive factor H aHUS Web database has been developed (http://www.fh-hus.org) that integrates genotypic, phenotypic, and structural information for mutations within human factor H. This provides a valuable tool for the interpretation of previously reported aHUS mutations, and provides prediction and analysis tools for new mutations. It will be extended to include mutations in factor I and MCP. Here, we describe how to use this Web database as a research tool, and indicate possible future directions depending on feedback from the clinical community.