Self-report tools for monitoring adherence can be useful in identifying patients who need assistance with their medications, assessing patient concerns, and evaluating new programs. The aim of this study is to test the validity of the Brief Medication Questionnaire (BMQ), a new self-report tool for screening adherence and barriers to adherence. The tool includes a 5-item Regimen Screen that asks patients how they took each medication in the past week, a 2-item Belief Screen that asks about drug effects and bothersome features, and a 2-item Recall Screen about potential difficulties remembering. Validity was assessed in 20 patients using the Medication Events Monitoring System (MEMS). Results varied by type of non-adherence, with the Regimen and Belief Screens having 80-100% sensitivity for "repeat" non-adherence and the Recall Screen having 90% sensitivity for "sporadic" non-adherence. The BMQ appears more sensitive than existing tools and may be useful in identifying and diagnosing adherence problems.