The Vaccine Safety Datalink (VSD) is a collaborative project between the National Immunization Program of the Centers for Disease Control and Prevention (CDC) and several large health maintenance organizations (HMOs) in the United States. The project began in 1990 with the primary purpose of rigorously evaluating concerns about the safety of vaccines. Computerized data on vaccination, medical outcome (e.g. hospital discharge, outpatient visits, emergency room visits, and deaths), and covariate data (e.g. birth certificates and census) are prospectively collected at multiple HMOs (initially four) and linked under joint protocol for analyses. Approximately 6 million people (2% of the US population) are members of HMOs participating in the VSD. The VSD has proven to be a valuable resource that has provided important information on a number of vaccine safety issues. The databases and infrastructure created for the VSD have also provided opportunities to address other immunization questions including vaccination coverage and cost-effectiveness. In a recent investigation of intussusception following rotavirus vaccination, the VSD methodology was expanded to include 10 managed care organizations. A cohort study was conducted that allowed estimation of incidence rates of intussusception and attributable risks associated with rotavirus vaccine. Published in 2001 by John Wiley & Sons, Ltd.