OBJECTIVES: To develop a standardized means for measuring the impact of medication adherence improvement initiatives in disparate pharmacy claims databases over sequentially overlapping periods of time. METHODS: The Adherence Index of Performance (AIP) tool was developed by combining Microsoft Visual Basic 6.0 with a series of SQL statements that transform pharmacy databases into a standardized format. Once standardized, a second series of SQL statements were applied to allow tracking of prescription activity levels compared to expected activity levels over sequentially overlapping periods of time. RESULTS: AIP detects trends in medication adherence relatively early after the initiation of adherence improvement activities. This represents a significant improvement over traditional methods for studying the impact of patient interventions on medication adherence and provides opportunities for early modification of adherence improvement activities that do not achieve their intended goals. CONCLUSIONS: AIP allows targeting of specific patients for adherence interventions based on persistence. Through use of dynamic filters, AIP can also identify health care providers and institutions where adherence improvement initiatives are most needed. Additionally, user-defined therapeutic groupings and intent to treat categorization allows AIP to generate accurate medication adherence information when patients are initiated on one medication in a therapeutic class but ultimately maintained on another medication within a defined therapeutic class. The Adherence Index of Performance provides a unique means for monitoring the impact of medication adherence improvement initiatives through analysis of pharmacy claims databases. AIP converts claims databases into a standardized format upon which a series of pre-defined analytical processes can be applied. This allows tracking of prescription activity levels for both new and continuing patients compared to expected activity levels over sequentially overlapping periods of time. It has been successfully used by a number of health care providers to monitor the impact of adherence interventions on patients' medication-taking behaviors.