BACKGROUND: Medication reconciliation is recognized as important, but no one method has been recommended. Research has shown that the most common medication reconciliation errors are attributable to omitted medications and doses. The pharmacy claims aggregator used in this evaluation is a private company that gathers pharmacy claims data from disparate pharmacy benefit managers into a secure repository (hereafter referred to as the claims database) under contracts with public and private health plans. A web interface for the repository can be used by subscribing health systems and health care providers to view patient-level pharmacy claims data to support patient care. At the time of this study, the claims database contained information from 5 public and private health insurance programs covering approximately 500,000 enrollees in Arizona.