Through a partnership that uses the data and other resources of three national and two regional health plans and insurers, we are conducting a multi-site retrospective study of the relationship between immunization with tetravalent meningococcal conjugate vaccine (MCV4) and Guillain-Barré syndrome (GBS) in adolescents (ages 11-21 years of age) over the 42-month period of March 1, 2005 to August 31, 2008. This collaboration has several features of a distributed data network, and so provides an instructive example of the feasibility of large scale population-based post-market evidence development.