Conducting matched case control studies can be difficult when matching on very large populations. We have developed an algorithm to identify a pair-wise matched case / control population for retrospective cohort studies. We have implemented this method using SAS code at two sites for the Vaccine Safety DataLink (VSD) project. This algorithm is superior as it works on certain extremely large populations such as those at certain Kaiser Permanente sites. The algorithm is thoroughly tested, robust, and effective for use on multiple sites projects to identify a pair wise matched case/control population, regardless of the dataset size each Medical Care Organization maintains.