Combined administrative databases are referred to as 'large linked databases' because of their relatively large size and the need for linkage of different data sets that were created separately from each other. Such linked databases have become popular in vaccine safety surveillance. Whereas their use offers some unique opportunities, their increasingly widespread use can also lead to wrongful linkage of vaccines to adverse events. We review the opportunities and hazards of using large linked databases for vaccine safety surveillance and propose some guidelines to increase the reliability of the outcomes. We also offer our opinion on the future use of large linked databases for vaccine safety surveillance purposes.